Forearm weight (used in Keyboard Playing)
A player can during key-descent, set free the weight of the forearm only, as a basis for the exertions of the finger and hand.
This comparatively light weight (of the forearm only) is suitable for light chords.
Where no great resonance is needed, and where the general musical effect is to be light and delicate, this technique can be employed. Realize, however, that this release of the upholding muscles of the forearm does not necessarily entail any actual movement or "fall" of the arm.
Classical Physics
Momentum (P) is equal to mass (M) times velocity (v). But there are other ways to think about momentum.
Force (F) is equal to the change in momentum (ΔP) over the change in time (Δt),
and the change in momentum (ΔP) is also equal to the impulse (J). Impulse has the same units as momentum (kg*m/s or N*s).
Note: It seems incredible that anyone could be so dense as to misunderstand or misinterpret my teachings as to the "element of weight". Other authors, however, seem to have believed that Matthay was referring to "movement", when, instead, he was referring to those invisible changes of state in the limb, from which arise the phenomena of the "weight being set free".
Arm off is not necessarily a Movement
Let it now be understood that when I speak of "Arm off" it does not mean that the arm should be raised-up
into the air! All I wish to convey by "Arm off" is that the incidence of this weight is here to be omitted; and, therefore,
that the arm should here assume or resume its poised condition through its self-supporting muscles coming into action.
Arm-on also not necessarily a Movement
Again, when I direct that the arm-weight is to be "lapsed" or liberated (set-free) this does not mean that the arm is necessarily actually to fall or move, but that the arm (for the moment), is to be left unsupported by its muscles (to the needed degree), so that this weight, invisibly supported and countered by the exertion of the hand and fingers, may thus serve as a basis for their action. Indeed, the full weight of the arm can be applied (and mostly should be) without the slightest outward or visible indication of its presence.