The phase after crush, the punchdown.
The yeast is on the skin. The sugar's in the juice. To get the two to interact and get spontaneous fermentation the tow need to be mixed together, so this is how we do that every day.
The skin has a habit of wanting to float on the juice, which it does very easily as fermentation gases build up.
This phase can last a couple of weeks. It's hard at first, but as fermentation tapers off, the skins are easier to push down.