Physiological Details of Piano Playing

The finger-tip is exerted downwards with the key during its moment of descent. The finger must reach the key slowly, but must then be exerted as required by the desired tone.

The Two Distinct Modes of Finger Use

One can bring the finger to the key and exert it with the key during descent, in two quite distinct ways or directions:

One can use it in a folding-in direction, as in everyday life, when grasping anything. It is the best and should be the most usual action at the piano, or
One can use it in the opposite direction, as an unfolding or opening-out of the finger. One sometimes uses this finger motion when playing at marbles and this type of motion is to be avoided.

The Condition of the Upper-arm Differs

The condition of the upper-arm differs in sympathy with these two distinct finger-activities, as we shall see later. This difference in the direction or mode of the finger-action and exertion (which may be invisible) becomes visible when you play with preliminary movements to the key that is, with a considerable preliminary raising of the fingers. In this case you will see that when using the "clinging" finger touch, the finger is in a somewhat flattened-out position when raised, and folds in when descending towards and with the key.

Tobias Matthay classified the following forms of touch which had been used prior to 1900, and these are to be avoided in modern keyboard technique.
Following Type of finger action is to be Avoided

When using the "thrusting" finger-action" (which is to be avoided), the finger is raised, is considerably bent, or even fully bent into an arch, and in this case the finger opens out when descending towards and with the key.
This form of touch is to be avoided.
When using the "flat" finger, the finger may be quite curved when it is down with the key. The fingertip grabs the surface of the key prior to setting the key in motion enabling you to apply the mass of the hand and forearm.