What is the black boxed area of the propulsive phase of the countermovement (CMJ) signal called?

It’s called the weighing phase and as the name suggests it is a phase of quiet standing on a force plate where the force plates quantify your body weight.

Does this phase serve any purpose?

It serves a few purposes but I will talk to one here. What can you see in the signal?

Well, the signals are from dual force plates. The black signal is from both legs combined, the blue and orange signals are from the individual force plates for the left and right leg respectively.

Dual force plates have many advantages, and you can see one right from the outset, a signal of an athlete cleared to return to play after ACL reconstruction. If the person on the force plate was evenly balanced on both feet, the signals would be overlapped. However, in this athlete you can see there is a weight shift, the athlete is unloading his left leg i.e. there is less force coming through his leg whilst quiet standing.

So what does this all mean?