The arrow is moving forward but the tips of the blades, as they open are not moving forward. They "grab" the target, and open as the arrow and broad head body continue forward. When the blades are done swinging back, the arrow has moved forward an inch or two - the blade tips are still outside the skin. Not untill they are done swinging back, do they move forward with the arrow and create that hole.
Just a side note to the main topic, but:
Is that not EXACTLY why so many people think mechanicals 'outpenetrate' or penetrate just as well as fixed blade heads? They shoot a Muzzy and a Rocket into the same block of foam and there is the same amount of shaft sticking out of the foam. So, the Rocket obviously penetrated just as well as the Muzzy. Right?
Wrong.
If you're talking about penetration of the insert, then yes, but what about depth of penetration to the back of the blades - the full cut diameter of each head - the part that really does the work? The back of the blades is a half inch IN FRONT of the insert on the Muzzys. The back of the blades are a good inch BEHIND the insert on the Rockets. Even though there's just as much arrow shaft sticking out of the foam on the Rocket shaft, the depth of full cut is deeper with the Muzzy.