Had an experience yesterday that I haven't felt since the old days on a Windrush clubman with the old triangular sail.
Was using a 7.4 freeride sail, which was way overpowered for the 20 minutes that the wind actually blew for.
Found it extremely difficult to bear off, being forced upwind.
Also had the sensation of the leading edge of the sail wanting to drive down into the water on the windward side.
It felt like I needed to put all my effort into pushing the front of the sail up, rather than the usual feeling of pulling the sail in.
I know that it was predominantly because I was overpowered, but is there something you can do to help get yourself under control?
I found that by moving the harness ropes further back made it easier, but not totally solved.
I've had this happen too.
I think what happens is that the flow separates at the mast on the windward side, so you have a bubble of air inside the belly of the sail. The wind is 'backwinding' the entry of the leeward side of the sail, pushing it towards you.
Two ways to counter this:
1: Put more outhaul on. This narrows the angle of entry, making the leeward side less prone to backwinding.
2: Sheet in...
I know that when you're overpowered sheeting in is the last thing you want to do. But it should stop the massive twisting moment that you experienced.
All cambered foils have rotational moment. The more the camber the greater the moment. On a plane it causes the nose to pitch down (hence the tail plane) on a sail you can think of it as back hand pressure or a tendency for the clew to rotate away and the mast towards you.
The rotational moment changes as the angle of attack (of the wind) changes. The lower the AOA the greater the moment. This is worse for windsurfers as if you sheet out in gusts the AOA lowers and the moment/rotation increases.
To solve this prob... don't sheet out (Slowies technique) or change down (my solution) or flatten the sail (symmetrical foils don't have this problem)
More downhaul, lower your boom, close reach into the wind, or bear way off, square reaches are the hardest when way over powered.
Angle the mast more across the wind rather than having the the tip of the mast into the wind.
The only thing to watch out for is that you are now hanging onto a wing in launch position.
this is the draft of the sail moving aft,its the classic overpowered situation as you suggested. sails have evolved in terms of full battens,tapered battens camber inducers etc all designed to stabilise the foil shape over the years. the flattening of the sail may seem like the best solution but its a double edge sword..its good for flagging the leach and reducing sail area but it also flattens the camber out and makes it easier for the draft to move back with the gusts and then forward with the lulls,causing the instability.
apart from changing down my way of dealing with it is to adopt the brace posistion
(lock out the arms) grit the teeth,HTFU and then run for calmer waters when no one is watching! 
i can remember way back when stocky rigs (windsufer soft sails with leach battens) where all there was....most sailors had harness line fixings a couple of feet apart...this was in effect a way of balancing the shifting draft or centre of effort...when a gust hit or it was windy you could hook up towards the back of the boom and slide back and forth to cope with the changes.
now i see dakine do a 1 fix harness loop! (prob see a few more broken booms with these!)
