You'll find some explanation here : Article Freeing the Foot

+ https://www.frontiersin.org/articles/10.3389/fspor.2019.00046/full

And here (conference paper) : https://bjsm.bmj.com/content/49/Suppl_1/A6.2

As far as I have learned, foot shortening exercises (also referred to as short foot exercises) are a form of sensory-motor training that activates the intrinsic muscles of the foot and actively forms/supports the longitudinal arch and the horizontal arch.

According to the principle of progressive overload, short foot exercises can be performed in a sitting position (with the hips, knees, and ankle at 90º of flexion) and subsequently performed in standing positions (a single-leg stance, thereby providing bodyweight resistance).

Patients are required to pull the head of the first metatarsal bone toward the heel without bending the toes and maintain the state for 10-20 sec (note: the forefoot and the heel should e prevented from being lifted off the ground and smoothly induce the flexion of the head of the metatarsal bone). Patients may need to perform 10-15 repetitions/set, three sets/session. Sessions should be repeated thrice/week in a 5-8-week program.

For more details, take a peek at the studies below:

Article The effects of short foot exercises and arch support insoles...

Article Short-Foot Exercise Promotes Quantitative Somatosensory Func...

Article How to Evaluate and Improve Foot Strength in Athletes: An Update

Good luck!

Thanks Dr. Ragab K. Elnaggar for your answer.