the "up" and the "down" of feet
The feet pick up, before they step down. Up down. Maybe they pick up on an in-breath and put down on an out-breath, but not necessarily. But they always go up before they go down. The back foot presses down so that the front foot can be picked up.
Then the front foot lands to pick up the back foot.
2 movements in each leg for each step. 4 movements to take two steps.
And I'm starting to sound like the beginning of Alan Partridge's autobiography, so I'll leave it there.