The limbs that don’t fit.
I teach many different patterns in drumming. Some are fairly straight forward and involve three limbs. Some are much more intricate and ask for odd combinations.
I’ll name those odd combos: something on the right side of your body falling exactly at the same time as something on the left side. Very difficult to do. And that includes trying to make the left hand falling with the right hand. Always some amount of flams for the inattentive player. And that’s the easiest one. The first one any drum teacher ask a beginner student to think about.
But there is a big one that needs to happen for intermediate drummer and above: the left hand with the right foot. Bass drum and snare together. Oh, of course, if it is in a disco beat and the brain has the time to prepare, to compute the operation, then it is not that hard. But give some kind of 16th notes combination to the hands while the player read something on the right foot, and you will see the tears come to his eyes.
I remember the very day when I was confronted with that impossible equation. I thought I had reach the top of complexity in drumming. Nothing after that could be as difficult (Boy, was I wrong!). I remember trying to de-rationalize it: “Come on! This is not a pattern. I’ll never use that in a gig! (I did). I can stick to easier and simpler patterns, no one will ask for that kind of shenanigans (They did!). Anyway, pushing and pulling, cursing the names of all the drumming gods, I managed to understand how to play my right foot simultaneously with my left hand. I was even able to play the bass drum loud and the snare soft while accomplishing that feat. I earned my intermediate hat with great pain. And that’s all right.