All the strength that is needed for a sure, firm attack will be provided simply by allowing each finger to drop into the key and by lifting it free of the key after playing.
The weight of the finger, as it is dropped, will give a clear, strong tone. And most of the physical energy applied is used for lifting each finger of the keyboard.
Stage 4
When finger strength and independence are developed to the stage that you feel a definite division of labour at the knuckles, so that, from the knuckles on everything happens automatically and effortlessly, the aim of scale practice has been achieved.
This leaves you then free to concentrate on the movement of the hand, the wrist and the arm.
All fluency, tone control and colour are dependent on this freedom, for they are achieved through the positioning and differing degrees of weight of the hand, the forearm and upper arm.
At this stage, too, slightly lighten your finger touch and play to the point of sound rather than firmly to the bed of the keyboard.
This means listening to the sound you produce and adjusting it to your ear, (instead of playing mechanically ). This takes the crudeness out of the key attack.