Agility, to be attained with ease, also implies accuracy in tone-aiming in this sense. Each key-moving impulse must be delivered accurately before it is too late to produce the intended key-motion (and tone), or else your fingers, hand and arm, will become mechanically locked against the key-beds, and will thus not only ruin all musicality, but will also inevitably hamper all ease in your progression across the key-board or so-called agility. Agility, therefore, depends upon rhythmical accuracy in this respect. In short, agility is a rhythmical act requiring the application and cessation of energy.

You will now understand why it is wrong to squeeze the key upon the "bed" beneath the point of tone production. For if you do so, you cannot "aim" your application of energy to the point of tone production. If you make this mistake (incorrect application of energy), your effort will be partly spent upon the key-beds instead of upon the strings. Hence the result thus obtained cannot represent the effect you intended and your playing will sound un-musical, because the result is not what was intended. "Key-bedding" also tires your hands and fingers. Likewise, it prevents agility, since it impedes your passage across the keyboard and prevents your execution of staccato, since the key cannot then be free to rebound, as it should be for Staccato.
The mechanism of playing demonstrates the supreme importance of early training in the subject of physiological mechanics. It has been proven that agility itself, and all the contrasts of tone and duration which enable us musically to express ourselves through the piano, depend immediately on the proper fulfilment of the laws of touch.

[1]first species:: Movement which originates from the hand knuckle (first-phalanx and metacarpal bone)