When I look at my weaknesses, the number one item on that list (according to me anyway) are polyrhythms. I understand how to calculate how the beat is divided but I struggle to make it work. So I addressed this Gap via Professor of Music Youtube.
For 2/3 polyrhythms, there are two notable videos which I am going to place here:
What I like about this one is it provides a scale exercise for making your fingers work on the concept.
Denis Zhdanov has a very useful video here also.
Currently I am working on these away from the piano and I’m interested to see that I can manage them away from the piano.