The so called "nulling" theorycraft:

It's a macro, that you can easily google up or even find on GameBanana.
If you can't easily find it, just stop reading here, because you're a troglodyte and you won't understand what's written below anyway.

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IMPORTANT INFO
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Stuff below was written 21.01.2018.
I don't care if you got "caught" and banned on some shit server, by an admin or owner(sic!), who probably doesn't even understand what
nulling is, on a server with an outdated timer and/or anticheat coded in 2012, wich most likely isn't even a legit copy of it.

Even if you got banned for that in late 2015, still, don't cry. It was 2 years ago.
Mistakes have been made. Mistakes have been fixed. At least on servers, where the actual timer/anticheat is being taken care of by an
active coder(s).
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1. How does it work?
    - Makes you never overlap the strafe keys. Doesn't matter if your strafe keys are bound to A/D, F/G, 1/2,
    V/N or whatever key you find on your keyboard.

2. What is overlapping?
    - If two or more activities, subjects, or periods of time overlap, they have some parts that are the same.
        Source: https://dictionary.cambridge.org/dictionary/english/overlap
        In this case, the subjects are strafe keys.

3. How does that give an advantage?
    - If you overlap a strafe key mid-air (eg. strafe key left, mouse movement right), you lose your velocity.
    - If you do not overlap the key mid-air (most people, if not all of us humans, overlap keys by a tick or two, or even three)
    over nine thousand times, you have a macro.
    Anyway, it's obviously possible to get perfect key changes without any overlap. Loads of players can get that.

    - If you have some speed mid air and do not press any strafe keys at all, but move your mouse.
    You don't lose velocity at all, but you don't gain either.

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    tl;dr:
        - Overlap a strafe key mid-air: velocity loss.
        - No overlapping of strafe keys mid-air: velocity gain.
        - No key press mid-air at all: nothing happens to the velocity.
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    - "Lol, this is not covering all aspects of nulling and not nulling"
    Yes, I know, Sherlock. This is just enough to understand the basics of how it gives advantage over people playing without any
    3rd party software/macros/put-whatever-you-want-here.

    - "I have K120"
    I have Dr Pepper and a soft pack of red West on my desk.

    - "Hey, you didn't mention 'idling'"
    Yea well, the point of nulling is to get rid of overlapping, not getting rid of no keys pressed while strafing. Why don't I go into
    details about idling/delaying strafes? Because this text is not about that.

    - "What about other styles?"
    Same shit for pretty much all styles. No, not for w-only, a-only, d-only and s-only.

    - "Can you tell wether legit, good player, or a player with null config?"
    In 99.9 cases yes.

    - "How?"
    Their sync.

    - "I don't have a macro and I always have sync1 and sync2 the same."
    No, you don't always have it the same. You usually do tho. Other syncs are different, I can assure you.

    - "What are other syncs?"
    Sync3 for example.

    "What's sync3?!"
    - That's one of the spells that only us, wizards, know. Such knowledge should never not be passed to muggles.