4/5 trills is one of the pianist's biggest fear (at least for me

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I once attended a concert with Zoltan Kocsis, the famous hungarian pianist, and he played the last sonata by Beethoven, opus 111. In the last movement, there is a famous part where you have to trill with fingers 4/5 while playing a melodi with the other part of the hand. He played the most beautiful trill I have heard, with fingers 4/5! So it is definitely anatomically possible! I have to admit that was one thing I remembered from the concert, because I was a young student and interested in technical things. (But the concert was very good, beautiful playing by a great pianist).
For the question of how to manage these trills:
I was surprised at the lack of movement of arms, wrist etc. All of the work was being done by the fingers. The only difference to my normal stance was a slightly raised wrist.
I don't seem to use much weight with these trills - the action is closer to running than to walking, relying on imparting even trip-hammer impetus to the key rather than a connected motion.
I believe that Thalbergmad and Ted are correct when they notice that the arm doesn't do much work in these kind of trills. For example, with 1/3-trills, you do almost all the work with arm rotation. But with 4/5-trills it is actually the fingers working. That seems incredible, since these fingers are so weak.
My theory is this: The 5th finger is the smallest and has a weak muscle, but has actually quite good independence and a stable structure. It can be trained to be as independent as the 2nd and 3rd finger (or almost). The 4th finger by contrast is quite strong, but lack in independence and stability. If you try to move the knuckle of the 4th finger, you will notice that it is quite loose.
If you are going to do 4/5 trills, you need a quite strong 5th-finger; it has to be trained. It can be trained quite well and can achieve great independence. The 4th finger will not be independent, but it is not necessary. At the concert with Kocsis I noticed that his 3rd finger was lifted high when he trilled 4/5. This happens automatically, because else you cannot move the 4th finger wich is connected to the 3rd. But the 5th and 4th fingers are quite independent of each other (if you have trained fingers). So the movement is: The 5th finger will move in an agile way, and the 4th will move almost nothing, and feel "connected" with the rest of the hand, while the 3rd finger is raised.
I also have noticed that these trills are somehow easier when the hand is stretched, like when you take an octave with the thumb and 5th finger. Maybe because the slightly tense hand creates more stability for the loose 4th finger. Coincidentally the cases where you need 4/5 trills are normally also where the hand has to stretch out, so this is fortunate.
This was more difficult to explain than I thought, but I hope you understood me.