Ok trying to look at how i think this is working - correct me if I'm way off.

The band in his first position forces him to keep his centre of gravity over his feet. If he began to go headfirst his feet would come out from under him.

If we run without bands we probably adjust our strides etc to allow us to throw our bodyweight all over the place. This is bad for anyone because of the injuries that can occur from leaning too much one way and putting stress on weaker or overworked areas. Also posture would become bad and that leads to a whole range of back and spinal issues.

This is bad for a boxer because it means you are more likely to have momentum you can't control and will find yourself falling into your opponent, over-reaching etc etc.

Also not allowing yourself to lean too much forward allows you to increase your cadence and run efficiently. Top sprinters do not lean forward. Increasing cadence is generally accepted as being the most efficient way to improve your speed.