David, the forward motion or energy from the arrow pushes the blades back and in the open position. As long as the arrow continues to move forward the blades remain open.

I was trying to recall all of the mechs. I have used over the years on deer...Let's see, snypers, various models of Rockets, Wasp Jackhammer, Marden, Gametracker Firstcut, Triska, and probably some others that I can't remember.

I can honestly tell you that none of the heads I used ever cost me a deer. Certainly Mechs. aren't for everyone but as mention those that qualify energy wise and tune their equipment, the mechs. will do a number on deer sized game.

The head that did the most tissue damage consistantly of all the heads I tried was Rockets Hammerhead 3L. Four of the five bucks shot with this head were complete pass throughs. Most fell over dead within view. Huge holes and generous blood trails were the norm. The heads look like blades from a blender when they were done, but they always did a number on the deer.