Road descents- like most everyone else, cranks level, either foot forward. Mtb- if descending in switchbacks or across a face (uphill on one side, drop on the other) I put the downhill side crank arm forward, to avoid rocks/roots/anything away from my uphill foot so that my line isn’t forced toward the “drop” side of the trail. Also, if I have to stop fast I find it easier to quickly stand on the downhill pedal this way and unclip the uphill foot to lean uphill as I stop, rather than trying to put a foot down over the edge of the trail. Either bike- outside foot at 6 o’clock in corners, a few quick pedal strokes, back to level, or the next corner... repeat til the finish line.