SetActorVelocity(int tid, fixed velx, fixed vely, fixed velz, bool add)
Sets velocity of things tagged tid. Velx/vely/velz are expressed in "fixed-point" mode, i.e. with decimal point changing the actual value (e.g. 10.0 is equivalent to 655360).
If add is nonzero, it causes the velocity vector to be added to things' current velocities. Note that monster movement (which has a characteristic jerky pattern) does not count as actual velocity.