Prone: While in prone position you can only move your half movement speed.
Hit the Deck: Half action, go into prone position. You may couple a dodge reaction with Hit the Deck for no additonal action cost. (you can dodge and go prone at the same time)
I have a question also. Did headshots deal 2x damage in older editions of 40k rpgs? I thought it did in Dark Heresy but I don't see anything in Only War about it. Is that rule recommended? I feel like taking a shot to the head should have a little more gravity.