Tobias Matthay used the word relaxation to suggest the sensation of free playing, for relaxation was the opposite of tension or rigidity. However, there can never be total relaxation, for a muscle cannot be isolated to relax it. Every muscle must be in partial contraction, for when muscles are totally relaxed, inertia^[1] is the result. I use the word free, for it is a better word and is more descriptive of what is required in keyboard technique. Free playing is a result of coordinated movements and coordinated movements minimize energy to maximize results.

The tendency of a body to keep moving once it is set in motion results from a property called inertia. You use inertia when you try to get ketchup out of a bottle by shaking it. First you start the bottle (and the ketchup inside) moving forward; when you jerk the bottle back, the ketchup tends to keep moving forward. The tendency of a body at rest to remain at rest is also due to inertia. You may have seen a tablecloth yanked out from under the china without breaking anything. The force on the china is not great enough to make it move appreciably during the short time it takes to pull the tablecloth away.

It is important to note that the net force is what matters in Newton's first law. For example, a physics book at rest on a horizontal tabletop has two forces acting on it: an upward supporting force, or normal force, exerted by the tabletop (see Figure. 2a) and the downward force of the earth's gravitational attraction
The upward push of the surface is just as great as the downward pull of gravity, so the net force acting on the book (that is, the vector sum of the two forces) is zero. In agreement with Newton's first law, if the book is at rest on the tabletop, it remains at rest. The same principle applies to a hockey puck sliding on a horizontal, frictionless surface: The vector sum of the upward push of the surface and the downward pull of gravity is zero. Once the puck is in motion, it continues to move with constant velocity because the net force acting on it is zero.

The condition of our muscles must arise in strict response to the felt resistance at the key surface. Muscular sensation arises purely from resistance opposed to the muscular exertion. Even the slight muscular exertion of the finger that supports the balanced hand on the un-depressed key is enough to excite the sensation of a muscular response. An equilibirum of balance of the fingers between the hand and key renders this sensation.
We must not mistake the mere sensation of touch (or contact with the key-surface) for the required sensation of resistance. The act of observing key-resistance, also induces us to pay attention musically. For we cannot attend to the key, and choose what to do with the key without constantly referring to our musical feeling, judgment and interpretation of the muscial score.

[1] Inertia: The resistance an object has to a change in its state of motion.