Also called vertical vibrations or motor impulses*. This term corresponds to an alternating between placing and removing the motor apparatus on the keyboard - see Neuhaus’ bridge and crane. The frequency of this movement is very rapid, but the vertical vibrations of the arm are extremely small.
*Czesław Sielużycki - “The pianist’s hand” p. 261 and 305-306, Polish Music Edition, Cracow, 1982
Of course, I do not mean that one must merely “shake the shoulder” to play correctly. The truth is closer to the opposite: it is correct playing that brings the impression of shaking the arms when performing certain very well defined technical movements. At the most, this shaking can be gradually stimulated principally by practicing staccato exercises or by doing gymnastics away from the piano (cf. exercise n° 2).
For more information, please refer to this chapter.