nyiregyhazi: The joint goes back away from the hand. Most people's are straight when extending the hand muscles - my thumb joint naturally bends at the top joint so the thumb hyperextends even more. I need to explicitly engage muscles in my thumb/hand to keep the thumb straight when flexing my hand. Here are pictures. The first one is my hand resting on the keys with my hand muscles slightly opened. The second one is the same, but consciously having to straighten my thumb.
I think it is evident that the second picture is what should happen for better technique, but it one that takes introduces a small amount tension compared to my "default" position.
Sorry for the trill hijack - it is tangentially related since it does affect my thumb trills!
Looking at the pictures, I feel 99% sure that it's actually the severe bend in the joint closer to the hand that you should be concerned by- rather than by the extension in the end of your thumb. The second picture actually looks way more troubling to me than the first. I had a student once, who had a very similar looking thumb. Initially, I tried various gripping ideas, but his thumb always had that same awkward looking bend. Apparently it's an anatomical thing called a "curved thumb". However, when I later tried getting him to concentrate on lengthening out his thumb under the palm, it turned out that his thumb could function just fine. The only problem was that he was fusing the joint near to the hand into a cramped position, but when he instead began to lengthen his thumb, there were no more strange looking angles. It just looked like any regular hand. Since then, I rarely teach students to grip inwards with their thumbs at all. It's much simpler when actions are based on lengthening than when they are based on trying to squeeze inwards. You may later need a tiny inward action in the tip alone, but first I'd work on lengthening the thumb out- not closing it up into a stiff position.
I think the big issue is that if the thumb starts as curved as that and relaxes, you need severe efforts to keep it balanced. If it were relaxed, it would get thrown around all over the place. When you strive to lengthen it out (not only at the tip but throughout the whole thumb- feel the thumb's movements beginning right back at the wrist), it's very simple to keep it balanced. All you need to do is maintain a tiny activity to remain not stiff but "taut". When it starts curved, it takes a lot more effort to keep yourself stable. It can only be done with real stiffness.
I've done various thumb exercises- for actions not of closing up the joints but of lengthening them:
https://pianoscience.blogspot.co.uk/2011/11/action-and-reaction-in-practise-part-i.htmlCan your thumb lengthen out like that, or is it not possible to do so?
Also, there's a book by Alan Fraser that is called "all thumbs" that I'd definitely recommend. He calls what you do "inversion"- where the thumb tries to raise itself up to the same height as the knuckles, rather than go down and under the palm. I'd try not to think of thumb as being something that just sits at the side of your hand, but rather as something that goes down and
under it.