When foiling we can allways see how water raise on one side of foil and splash,what side of foil do that,windward or leeward?

Both

Mostly downwind

Both.
As water releases from the mast of the foil, it gets hit by a little wind and gets blown downwind of it's release point.

The good thing about foiling is it opens up a whole lot of stuff that'd never enter your head on a slapper. I thought it might have something to do with the Coanda effect. So I googled that, nah. And then got side tracked onto this one the youtube threw at me.