That is what has me so curious, although a mechanical is said to open upon impact, even as fast as they could open....the arrow is already moving at a rate of speed, depending of course on how fast the bow shoots that arrow. Oh well.
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.