I use "constant" to make keyframe animations. But when I converted the animation curve to "bezier". The animation in the middle frame looks very strange. The rotation of the hand will be very exaggerated. This happened when I used Maya to make animations, but there is a command in Maya that can be used to optimize these animation curves. But I don't know if there is such optimization in Blender.
2 Answers
I think, what you are looking for is "Smooth keys".
Just select the keyframes, which you want to smooth in "Graph Editor" and then hit CTRLO or use this menu:
For even smoother keys tap it again...and again...and again ;)
Looks like an euler rotation interpolation issue: you can try to use a different euler sequence, other than euler xyz, but probably you will have to translate your hand animation into quaternions.
https://www.youtube.com/watch?v=SZE_KOjreAs&list=PLE211C8C41F1AFBAB&index=12