We practiced our hip rotation practicing hamni and shomen positions in zenkutsu dachi, adding blocks age uke and soto uke, followed by gyaku zuki. Although the hip and arm move together, we have a feeling of the hip driving the hand to ensure the power is coming from the floor. Keep your shoulders down and relaxed. Ensure your head remains looking forward and that your front knee doesn't move/collapse.
We practiced whipping our arm forward with the hip driving it forward without power, before adding tension at the end to generate power. The relaxation and tension at the end along with using the whole body and hip rotation is how we generate power. The whole body tenses, not just the arms. This creates a strong, fast and sharp that's essential for an effective strike.