One of the methods I’ve had success with for training the left jab/right cross/left hook boxing combination — and for training the left hook specifically — is to have the student hold their right glove to the focus mitt and freeze for a moment before firing off the left hook. The idea is to have the student think about their footwork, weight distribution, and hand position before they whirl around with the left hook. Too often I’ve seen students develop extremely bad form in the left jab/right cross/left hook combination because they go too quickly and rush the motion. You should slow everything down and work on speed and power later. First must come proper form, which means proper body mechanics.