In the hitdef it could be one of many things
1) pausetime = 0,x
x = as long as you want P2 to pause before falling
2) If could be the air.velocity & ground.velocity that is pushing the character down, if this is the case you would want to have 2 hitdefs for the move, one for the first few hits with air.velocioty = 0,0 and a second hitdef with air.velocity = 0,5 or something.
3) you could add "p2stateno = 900" to the hit def then make a special state "900" with:
[Statedef 900]
type = A
movetype= H
physics = N <------------ no air falling
anim = 900
see, now [statedef 900] is what happenes when P2 gets hit by your attack. This is used for throws all the time. You can make P2 do anything in the world by using this (like how Michael Jackson makes opponents do a dance)
Hope one of these helps.