Comebacks is simply a way to travel far distances in Toribash. It's seen especially in wushu/lenshu mods, where you might need to catch up to an opponent with higher score. It was originally thought up as a counter to the runners and tripoders of early wushu. Nowadays it's part of the mod.
Comebacks are done by putting a contracted joint against the ground and extending it, pushing you towards the opponent. It's hard for beginners to get the right angle. Getting the wrong angle can thrust you upwards, launch you past your opponent, or launch you further away from your opponent.
It's generally seen as rude not to comeback in wushu and lenshu mods, especially if you're leading in points. However, it's in no way forbidden.
Setting up is an essential part of comebacks. They are typically done by maintaining a pose where one or more joints are contracted and ready to extend, pushing you towards your opponent. The difference between a setup and a comeback is that a setup is always done without movement.