Jumping - What happens if you pull your back/front brake in the air?
If your wheel is spinning hard in the air, and you apply a brake hard, the rotational momentum has to go somewhere.

However your wheel is rotating forward, and stopping it will make your bike rotate so the front drops.  Often that's not what you want.

Also the time available is really small and your reaction time shortens the effective window too.

If your wheels are not spinning fast then there's no stored momentum there so applying the brakes will do nothing.



The question you didn't ask is "What happens if I land with the brakes hard-on?"   and that generally means badness if you land with any forward momentum.  

The front wheel will be locked so it will either make the brake slip or it will make the tyre slip on the ground, starting a front-wheel washout.  There's also a chance your front wheel will turn sideways which will lead to a taco  or a complete OTB (Over The Bars).  Doing this is most likely to put you in hospital or worse.