Apparently there is no such thing in Toribash, but I guess it's been tried to be emulated somehow.

An example of this mechanic.
2NoSlowdown.rpl
2Slowdown.rpl

So, what/where are the exact mechanics behind this, how can I control it?

That's not air resistance... I don't think it is anyways.. it just seems that the center of gravity for the tori is slightly behind the tori itself (specifically below and behind the glutes slightly), so when you fold the feet like in 2Slowdown, where they're behind your butt, your whole center of gravity is transferred to your whole body, causing your body to catch the gravity acting on all of it.

There's probably a certain point that your feet can't pass but I made a general rule for myself that you shouldn't bend your knees passed 90 degrees if your hips are extended, otherwise that weird thing occurs and you just lose all your momentum..