Imagine a board floating in the water. It takes up some space, so if you suddenly took it away, there would be a board-shaped ‘hole’ left in the water. Of course, the water would quickly fill this hole.

Now imagine a board moving through the water (non planing). Obviously, it has to push the water in front of it out of the way. But just as importantly, it will leave behind a hole in the water where it used to be. This means that there will be a ‘pile-up’ of water at the front of the board (which means higher pressure) and a ‘hole’ left behind your board (which means lower water pressure). Both water and air rush in to fill the hole left behind, so what you see is an area of ‘white-water’ (water mixed with air) at the back of the board.
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