Supination is a movement that occurs toward the end of the stance phase. This is a period where the foot is now becoming more rigid and preparing to propel the body forward. A supinated foot is noted by characteristics and positioning that are the opposite of pronation.
The foot leans outward and the arch height increases. Muscular contraction assists significantly in this movement toward a more rigid foot type. If the foot is flexible during this phase then injury can quite obviously occur. For this reason the joints of the midfoot and forefoot in particular need to become more tightly packed. A mechanism called Windlass also comes into play whereby the tightening of the soft tissues in the arch and flexing of the forefoot also assist supination.
See in the video below the way in which the left foot rolls outwards excessively? It is probably obvious that this foot is supinating abnormally, however it does highlight the grossly supinated position the foot assumes during weightbearing. Excessive supination or timing of supination can lead to lateral instability and more laterally located injuries.