How do I make a particle go a certain direction relative to the player or the normal of a collision during c_trace or c_move? I understand the x y z thing, but how do I turn a point into a velocity for a particle, or use the normal of a surface to make a particle go a certain way?

I have a blood effect that I want the blood to come tward the player, and away from the player when the enemy is shot. I also want a smoke-ish particle to come off the normal of a wall, when it is shot by a c_trace.