Well, that is not exactly a jump but a teleportation. Try this instead:
_jumpvel = 1
player setPos [(getpos this select 0),(getpos this select 1),0.1]
player setVelocity [0,0,_jumpvel]
And for a forward jump (you may change _jumpvel to any desired value for longer jumps, but the player may result harmed when landing:
_jumpvel = 2
player setPos [(getpos this select 0),(getpos this select 1),0.1]
player setVelocity [sin(getDir player)*_jumpvel,cos(getDir player)*_jumpvel,*_jumpvel]