A rotation is simply the movement you use in things like turning a door know or screwing in a light bulb. Technically a forearm rotation because the bones of the forearm are turning around each other, it sometimes referred to as a "wrist rotation".
The rotation is an incredibly important movement in piano technique, and you'll get a lot of benefit out of learning to execute it with supple arm motions. One way to get a good feel for the rotation is to stand and let your arms hang at your sides as demonstrated in this video. Think meatball on the end of a noodle!
- Start practicing rotation with fingers 2-5, probably with the interval of a fourth such as C to F. We don't use fingers 1-5 at first because this can lead to cramping the thumb in and putting a crook in the wrist.
- Remember: when playing with two fingers, you want a straight line through the middle of those two fingers that goes through the wrist and up the forearm. Alignment is important for this exercise! Imagine a point in the middle of the wrist, equidistant from both fingers, that you are rotating around.
- With a fairly quiet hand, find neutral position and initiate with a gentle drop onto the second finger. Then practice gently rotating between fingers 2-5. Be very careful that you are not playing from the knuckle joint but are instead rotating the entire forearm. It is very common for people to think they are rotating when they are actually playing from the knuckle joint. Of course in actual playing a "pure" rotation isn't usually necessary - the fingers will come into play - but we want to make sure for now that we can distinguish between these two motions.
- After you feel comfortable rotating between 2-5, practice between other pairs of fingers: 3-5, 4-5, 2-4, etc. Then slowly add the thumb in, being very careful not to use poor hand alignment.
If you find that you tense during this movement, you can pause on either finger and do a slow finger "pushup" to check for forearm relaxation.
Important note: Remember that in a rotation movement, the imaginary point at the center of the wrist should remain fixed and should not move from right to left!