Pronation = dorsiflexion + abduction + eversion
Supination = plantarflexion + adduction + inversion
A great way to understand it is to look at your feet while you rotate. As your tibia internal rotates, your arch flattens. This is b/c you are pronating. The talus is adducting and plantarflexing. This supinates the midtarsal joint wand unlocks the forefoot. By unlocking the forefoot it allows the forefoot to abduct, evert, and dorsiflex.
As you external rotate your tibia, you supinate. The talus dorsiflexes and adductus. This makes the midtarsal joint pronate and locks the forefoot. A locked forefoot is pulled back into adduction, inversion, and plantarflexion.
Now if you look at the gate it makes sense when these events normal occur. Just think supinate, pronate, supinate. You first supinate at heel contact, as you weight moves over the leg your foot pronates to adjust for uneven ground, and just before push off you supinate to lock the forefoot and aid in plantarflexion. If you look at how you shoe wears out it is like a C shape.
P.S. I hate biomechanics but I hope this helps.