The little finger is the weakest and shortest finger, yet its column is the widest. On the left it owns Q, A and Z; on the right P, the semicolon and the apostrophe, plus every edge key: Shift, Enter and Backspace.
q a z p ; '
Because the edge keys live here too, this finger is on duty in almost every sentence. All the more reason to get its movement right: the finger reaches for the key on its own, the palm does not rotate along with it and the other fingers stay home.
When the little finger tires, the reason is almost always that the wrists are resting on the desk and the hand cannot move freely. Lift the wrists and the work is done by the arm and shoulder rather than by one finger.
The drill mixes both hands, because that is how these keys turn up in real text.