The supinator is actually the most stable of the foot types. Even though there is imbalance within, all the metatarsals are peaking in their maximum load at about the same time. 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. 1 you can see that, starting with the 5th metatarsal (outside of foot) the pressure (GRF) is very high, in fact it is off the scale. As we move down we see that each metatarsal peak is very nearly under the one above. For the most part each metatarsal is peaking at the same time, this is the characteristic of the "supinator foot".
In Fig. 2 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 last point of interest is to notice the peak of the first metatarsal (the lowest GRF) in Fig. 1 and then see it again in Fig. 2, the GRF is actually raised through the principle of "fluid mechanics" in order to complete the balanced alignment.