Walking in-and-out

The walking in-and-out adds another movement to the number of movements available in piano playing. It involves the whole arm from the tip of the finger to the shoulder. Because of this, the pianist is able to move in and out between the black and white keys easily. Moving the whole arm in this way gives enormous power to the finger. It is easy for the arm to adjust to where the finger has to play to be in a position to generate unique sounds. When playing a combination of black and white keys, it is natural to play a long finger (2-3-4) on a black key and a short finger (1-5) on a white key when possible.
If a short finger is required to play on a black key, then the other fingers must move further in the depth dimension.
The fingers must never play in a straight line in the black key area nor in a white key area.
Moving in-and-out gives power and control and avoids the dreadful twisting at the wrist. It must be remembered that the movement in and out will take care of the leverage in that area. It is most important that the arm weight be over the top of each finger at all times, even when it is in a static position in a white key area.

Balance the arm by bringing it over the finger for every note. In any given passage, do only what is absolutely necessary and do not complicate the passage with unnecessary motions. If at times there seems to be a choice of motions in a certain passage, try the passage at maximum speed.
The motion that you are able to execute using a minimum expenditure of energy and correctly is the one you should select. Moving in-and-out takes in a great deal of distance and the black keys are never felt as obstacles.

The

1. walking arm,
2. forearm rotation, and
3. in-and-out

all deal with the study of motion, which is the "how to get around the keyboard" principle.

When the arm walks in, it will move much less when combined with an up-and-down motion of the wrist. This implies various wrist elevations depending on the nature of the passage being played. A combination of these many things brings a division of labor that can be worked into the smallest possible projection.