What 445 Supermag said. They tear, which will reduce cloting. This would be beneficial in marginal hits by making it harder for the trail to stop, and leaving more blood to fall. Slice your finger and it'll bleed; rip your finger off and it'll bleed a lot more. Stealthforce is the only broadhead company I've seen with serated blades. The front part of the blade is plain, and the back part is serrated. The effect would be the serration tearing up what has been already cut.

God bless,
Brandan