Screw Attack

Before going forward I will clearly define all of the special terminology needed to describe Screw Attack succinctly, then I will describe what factors influence the Screw Attack's behavior, then I will discuss Screw Attack's behavior in detail.

After pressing the B button a third time while in the air and looking and moving forward, you are put into a state which I will refer to as the Screw Attack animation or SAA. While you're in SAA you have a morph ball environmental collision box which is useful for doing SA secret worlds and you have a death radius around you that deals 36 damage per frame, enough to kill most enemies near instantly and very quickly annihilate most bosses (at least those that are vulnerable to Screw Attack type damage) in anywhere from 1-3 Screw Attacks.
While in SAA, if you press the B button a fourth time, under normal gameplay circumstances it will activate subsequent jumps which are useful for crossing gaps. This will be referred to as an Actual Screw Attack or ASA. It's worth noting SAA is a state while ASA is an action you commit while in that state. After doing an ASA, you are still in the SAA.
After SAA has been cancelled by any process (there are three that I am aware of), you will be put into a state I will call Cancelled Screw Attack or CSA.
Screw Attack's behavior is entirely determined by the current height of a specific variable, which I will refer to as the z axis

With that out of the way, the z axis can be changed by 3 ingame actions that I am aware of:
*The z axis is set to global zero whenever each world loads. Note that a world in this context is defined by which pak file they are located in, and the z axis is initialized whenever a pak loads. Because entering an elevator, dying, or warping in Menu Mod load a new pak, they reset the global zero. What's important to note is that entering the teleporters is not included on that list because the Dark and Light versions of each area are located within the same pak, meaning travel between Dark and Light Aether is able to retain the z axis. Here are the rough locations of global zero in each area:
In Temple Grounds, it's way above the map, about the height of the birds in GFMC compound
In Agon, it's about the height of Portal Site
In Torvus, it's just above the bottom of the elevator leading to Lower Torvus
In Sanctuary, it's about the height of Grand Abyss and Watch Station.
In Sky Temple, it's well below the map.
*Using Space Jump. Specifically, when you press the B button with Space Jump it sets your current height as the z axis. It's also the only one that's relevant in categories with Space Jump, as using Space Jump is a prerequisite to using Screw Attack in all cases. This is obviously not relevant in NSJ categories because you can't use SJ. However, if you want to experiment in Menu Mod for NSJ categories with an arbitrary z axis, you can give yourself SJ, set it, then take it away (that is what I did for this video https://www.youtube.com/watch?v=jOxrg-iZ3kY).
*Jumping off a Screw Attack wall. Specifically jumping off it after you've stuck to it; sticking to it and falling off has no effect. I'll discuss the conditions in which you can stick to it later.

When attempting to use Screw Attack, the exact conditions for when you can or can't ASA is determined by the z axis in the following way:
If you drop too far below the Z axis, the SAA will automatically cancel itself. I believe the cutoff for this is about the current z axis-5. This obviously prevents you from doing anything other than the easiest SA secret worlds.
If you are in SAA, pressing the B button too far above the z axis will cancel it. I believe that the cutoff is about the current z axis+5. This is useful when already in an ASA because it allows you to exit faster and it gives you more speed for SA secret worlds, but it also prevents you from doing an ASA if you are too far above the z axis.
Also if you bonk against collision or just land on a standable spot, that will cancel the Screw Attack.
I have not done extensive testing with memory viewer so I don't know exact heights for either of those. The easiest place to test this is in Portal Site as the global zero location is very convenient there.

When using an ASA, the maximum height is determined by the z axis. Because of this, in SJ categories it's useful to time your SJ so that the height is optimal for whatever secret world you're doing. The timing of the second B press is of far higher value than the timing of the third or fourth. The same is true when killing EI3 with SA; a low SJ allows the SA to more easily connect. When doing SA secret worlds, the fifth B press is important in some cases; an ASA can give you a large speed boost, and cancelling the Screw Attack directly after removes the risk of bonking, which makes some secret worlds much easier. The second B press is still the largest factor though. You can ASA a total of 5 times, but it's worth noting that after the fifth ASA, you cannot cancel it with the B button and you have to cancel SAA by falling below the Z axis or hitting collision. When doing an ASA, you are given a speed boost up to maximum speed, and then your speed drops off over time. I am not exactly sure what the relationship is between time and your speed, but rapid Screw Attacks are faster than spaced out Screw Attacks at the cost of decreased distance. In any situation where you want to hit the load trigger as soon as possible, like Watch Station Access for example, the optimal strat is it to do rapid ASAs at the beginning until hitting the load trigger, then stretch out however many ASAs you have remaining.

While in CSA, it's important to note that you cannot leave CSA and return to normal gameplay without landing on a standable spot. What this means is that if you are unfortunate enough to enter SAA while out of bounds, you're probably fucked basically. Don't ever use SA while out of bounds unless you absolutely have to and/or know exactly what you're doing (see Aerial Training Site or the transition to Unseen Way). What's really annoying is that unless you land on a standable spot, the game still counts you as in air when determining whether you B press should put you in SAA. The easiest way to avoid this is to look down. Looking up or down for some reason makes it so you can't Screw Attack, and in this case down is the easier and more useful one. I have not tested what the exact camera angle requirement is, but somewhere around 45 degrees away from horizontal is pretty safe in my experience.

When playing NSJ, in most situations you aren't going to be exactly at the z axis, and so an ASA is not possible. Using SA in these situations is sometimes referred to as an SA fizzle or SA flop. However I am not really a fan of this terminology because whether you are too far above or two far below actually causes Screw Attack to exhibit two different behaviors. If you are too far below, the SAA autocancels and you can't really do anything with it other than the easiest SA secret worlds and moderately increasing your jump distance. When you are too far above, you cannot activate ASA, but you can still stay in SAA for extended periods of time, which makes it possible to deal damage with Screw Attack and stick to Screw Attack walls. Generally speaking being too far above is far better than being too far below, but still nowhere near as good as actually being at the z axis. In situations where you are at the z axis, like in Dynamo for example, you have no direct control over the maximum height of the ASA which makes the the Luminoth corpse SA secret world much harder.

In order to stick to a Screw Attack wall, you need to be in the SAA. It doesn't matter how many ASAs you have left nor does it directly depend on how close you are to the z axis, but obviously SAA autocancels if you drop below z axis-5. It has been hypothesized that maybe there is a 1 frame window where you can connect before it autocancels, and if this were true then FPS skip in 4% and float skip in Max% NSJ would be solved, because you can climb the wall in Undertemple and you could jump off the wall in CHWT to set the z axis to a height that allows for Culling Chamber secret world. Unfortunately there is no evidence that such a frame exists because sticking to a wall with an autocancelled Screw Attack has never been demonstrated nor has any damage being dealt with an autocancelled Screw Attack been demonstrated, but I'd love to be proven wrong.

On the topic of Screw Attack walls, while raising the z axis with them is straight forward, it's not actually impossible to lower the z axis. If you have stuck to it, you can fall slightly before getting ejected from the wall. If you stick to the wall while as low as the z axis will allow, then jump at the lowest point possible, it is *possible* to very slowly lower the z axis. Using the walls in Aerial Training Site and in CHTW, it took several minutes but I was able to lower the z axis to about the height of the floor at the top of CHTW. Getting it much lower than that though I don't know how to do, so currently this hasn't solved FPS skip.

Currently in NSJ categories, the Screw Attack wall jump's effect on the z axis is used in one case, Sky Temple. The global zero is well below the map, but the Screw Attack wall allows you to climb up into Sanctum. You jump across, enter SAA and jump off once, and then this allows you to try again but this time you can ASA and then climb all the way up. If you climb part way up then fall down, you will be unable to stick to the wall because now you're below the z axis, and so you're forced to reload the Sky Temple. After successfully climbing Sanctum Access, the z axis is set to the the top of Sanctum, and this is what allows you to leave Sanctum after killing EI3. You can tell because if you get an abnormally high final SA wall jump, you'll have an abnormally high ASA after killing EI3. You can see this in my 4% pbs. In my 8:16 SS I had a low SA and had a very easy time getting out of Sanctum https://youtu.be/JPW_NBhpYjI?t=32800 and https://youtu.be/JPW_NBhpYjI?t=34900. Meanwhile my 5:31 seg had an really high final SA, which made getting out of Sanctum really obnoxious https://youtu.be/wuYjfgdzv-8?t=20280 and https://youtu.be/wuYjfgdzv-8?t=22280. Unfortunately it's also what prevents you from damaging EI3 with Screw Attack. After entering the arena, the z axis is just way too high to deal damage before SAA cancels itself. And even if you could enter the arena with a Z axis that allows for killing EI3, you would be unable to leave. This also is what prevents you from using the checkpoint after killing EI3. If you die, recall that it resets the z axis to global zero, which is way below the map in Sky Temple. This means that while you can stick to the SA wall in Sanctum Access, you have no way of crossing the gap in order to leave Sanctum as attempting to ASA will cancel your Screw Attack.