For me, whether I'm double jointed or not is a coordination thing. It's sort of like, if I'm unaware of, and "dead" in, my finger tips, and gripping the keys by flexing mainly the second joint of the finger (the joint 2nd closest to the fingertip) my fingers, or more specifically the first joint closest to the finger tip, collapse like hell. I have to ensure my focus is in my finger tip, and that I move mainly from there, pulling the very tip veeeery slightly towards me (allowing the knuckle joint to bend), and then my fingers get a nice arch and no longer collapse. So it's not about forcing the finger into the correct shape by tensing it, but rather moving it correctly (and not pressing down on it with the arm).
Alfred Cortot is quite clearly double jointed (I'm not talking about the use of flat fingers, but the instances where the fingers are curved yet the outer joints are collapsed). He was still able to have a virtuoso technique, until Parkinson's disease started taking its toll. Video of him playing starts at 1 minute.
Thank you for the replies... it's quite interesting. So I presume that being double-jointed might appear as if people are playing with weak fingers and not fully supporting them with their hand. I had a student yesterday whose finger 5 on the RH sticks out quite a bit. I tried to get him to do an exercise where he holds finger 5 on G, and tries to hold it as he plays underneath the pattern: C D E F E D with 1-2-3-4-3-2 etc. Every time he tried, he could play C D & E but the second he tried playing F, his G (5th finger) would release from the key - unable to physically hold it down. To me I thought it was just bad technique from his previous teachers, but he told me that it was a little sore to try and force his finger to do the exercise. Is this normal for most people who are double-jointed??? I'm FASCINATED by this only because it's something I've never really experience before.
I had a student yesterday whose finger 5 on the RH sticks out quite a bit. I tried to get him to do an exercise where he holds finger 5 on G, and tries to hold it as he plays underneath the pattern: C D E F E D with 1-2-3-4-3-2 etc. Every time he tried, he could play C D & E but the second he tried playing F, his G (5th finger) would release from the key - unable to physically hold it down. To me I thought it was just bad technique from his previous teachers, but he told me that it was a little sore to try and force his finger to do the exercise. Is this normal for most people who are double-jointed??? I'm FASCINATED by this only because it's something I've never really experience before.
The extensor muscles in the forearm, the ones that extend each finger, are interconnected, but when used gently, they can lift each finger, except the 4th, rather independently. However, if you put a lot of pressure on one finger, and it primarily resists that pressure by engaging the extensors, the other fingers will involuntarily lift. Finger movements "pushing away" from you, or pressing down with the arm on a finger like described above, can cause this. It could be possible your student is doing some kind of uncoordinated pushing movement with the 4th finger, rather than a gentle gripping movement.
What I have figured out is that in my case it is actually the need to curve the 4th for stability (to avoid middle and nail joint collapse) that is causing the 5th to curve as well. The lifting simply comes from the need to get it out of the way (in addition to help lift the 4th, which in my case only can left very little independently), since there's very little room between these 2 fingers and my 5th naturally bends towards and under the 4th when relaxed. When playing with straight extended fingers deep into the keys this does not happen. So it probably is related to the double-jointed thing or a specific hand shape.
It could be possible your student is doing some kind of uncoordinated pushing movement with the 4th finger, rather than a gentle gripping movement.