Explanation of 2nd part of yoshi clip

[From mraglar (3-Jan-2013)]

I've done some short research of the supposedly "random" factors that determine whether you'll pull off the second part of "the underwater clip with Yoshi" or not. I should start by saying that unfortunately there's really nothing you can do about it in order to increase the success rate, but I thought sharing the knowledge of why it works the way it does is never a bad thing.

Before I took a deeper look into this (I have no earlier TASing experience with this game) I was quite certain that this was an issue of having different vertical subpixels everytime you're going into the wall in the first place, but this turned out to not be the case at all. The reason it works the way it does is because of framerule based vertical physics when being under water. This means that your vertical speed can only decrease at certain frames, in this case every fourth frame.

So let's say that your vertical speed can only decrease when the frame counter modulo 4 is 0, for simplicity. If you're now standing on the ground and push the jump button for 1 frame at frame 1 it will take 3 frames before the vertical speed decreases while if you push the jump button for 1 frame at frame 3 it'll only take 1 frame before the speed decreases, and thus in the second case you'll reach a lower height than in the first one and your vertical speed will also reach 0 at an earlier stage in the jump. And reaching 0 as fast as possible is the important thing when executing the trick since you won't be able to mount Yoshi if your vertical speed goes upwards.

My tests showed (by holding down/left while unmounting Yoshi, don't know if you do it differently) that the trick works if you unmount Yoshi when the framecounter modulo 4 is not 0 - that is in 3 out of 4 cases. In the fourth case you'll reach that 0 speed too late into the jump so Yoshi has reached too high up when re-mounting him, otherwise Yoshi will be low enough to still be inside the wall. So the chance of succeeding is 75%.