Don't change their origin, only their velocity.
Code:
new Float:velocity[3];
pev(id,pev_velcoity,velocity);
velocity[2] = -1000.0;
set_pev(id,pev_velocity,velocity);
This should make them shoot upwards, still obeying the laws of HL physics (they'll stop if they hit something, and will eventually fall back down).
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3000 PTS