WhiteofHeart said..
I understood its the other way round. Because the sail is fixed, heeling it over to windward increases mastfoot pressure!
Look at a force diagram, like the one from the MUF treaty about sail forces:

Pulling the sail more to windward increases the "up" part of the sail force vector, so the sail "carries" more of the sailors weight. That means less of the weight pushes down on the mast foot.
It's quite possible that the mast foot pressure changes differently due to
indirect factors. The weight is transmitted onto the board over both feet and the mast foot. If the distribution of the relative amounts changes, then that will have a larger effect on mast foot pressure than small changes in mast angle. For example, a freeride foiler with an upright stance can have most of his weight on his feet, and very little weight in the harness. In that case, the pressure on the mast foot is mostly the weight of the rig.
In contrast to this, a foiler hiking out far over the water must have most of his weight in the harness, not on his feet (using arms will work for a short time, but still puts his weight on the mast foot).
So if a foiler goes from an upright stance to being hiked out on the water, without changes in the wind or direction, there's a pretty good chance that he'll also transfer more power into the harness (possibly without noticing it). That will have a larger (and opposite) effect on mast foot pressure than the mast angle changes - increasing mast foot pressure, and pushing the nose down.
What you want is stable mast foot pressure as the mast is leaned to windward, so you'll have to carefully limit the transfer of weight to the harness lines. That pretty much means you'll have to have most of your weight in the harness before moving the mast to windward - which in turn should require quite a bit of body and ankle tension.
I can't speak for you guys, but I know that when foiling, I tend to have a lot more weight on my feet than when windsurfing. That's probably partly because I mostly foiled on big freeride foils and small sails. I've switched to faster foils, but changing these habits will take me a while. After perhaps a dozen sessions on the faster foils, I'm nowhere close to the harness commitment I have when windsurfing on slalom gear.