This material is increasingly prevalent in the context of the physical preparation, is an excellent simulator of support to improve the muscle power of the legs in the movement.
Simple to fix, thanks to its belt velcro, it does not require any special skill to use it. Hardly, you turn to run as the parachute gongle and opposes a resistance proportional to the speed of the race.
Imagine that you run into a staircase. It is exactly this that simulates the parachute. Hence its appeal.