The pronator is the least stable of the foot types. In this imbalance one or more of the metatarsals are peaking out of sequence with the other metatarsals. The imbalance is recorded in force called "ground reactive force" (GRF). Each metatarsal has to load in order to perform the "task" (of supporting, balancing and pressing to propel the body).
In Fig 3 we see how the 5th metatarsal (in pink) is "peaking" early, that is compared to the peaking of the other metatarsals the 5th is "out of balance". This causes the other metatarsals to work harder thus producing a higher GRF.
In Fig 4 we see the same foot (wearing an SDO in the sock for test purposes) with the arch filled with a prescription volume of silicone fluid. Notice at the top with the graphic representation of the foot how the surface pressure has been
reduced. Now look at the GRF and notice how significantly it has been reduced. Also notice how each peak of all 5 metatarsals are directly under each other. The peak of the 5th metatarsal has come into balanced alignment through the principle of "fluid mechanics".