Imagine it like a force field. I don't think it's x y z like the above poster, but I could be wrong, I don't use it ever. I think the first param is time it is active, the second is radius size, and the third is the force or velocity that acts on the player.
I had an incident with this where a guy named fungusumungus came on my server, got trust from someone, built a looping radius impulse brick with a large radius, and it took me forever to find the brick, or figure out who's it was.
