A spring clip retains the blades in flight, eliminating rubber bands or o-rings. We found the spring-clip tension to be too tight, though, impeding the blades' deployment. We're sure this is why this head produced the worst penetration of the mechanicals--a dismal 5 ¼ inches. On the bright side, the head is rugged, receiving high marks for integrity.
We are confident that NAP will tweak their blade retention system and eventually have a winner with the Killzone, but it's not there yet. ($40/3-pack, plus practice head; newarchery.com)