In the musical language legato - Italian word for tied - indicates that notes are played smoothly and connected.
That is, the player makes a transition from note to note with no intervening silence.
Playing legato does not just mean playing without taking your fingers off the keys between one note and another.
Instead it means finding the relationship between intensity and duration of each sound, so that one hardly notices the decrease in volume that is typical of a percussion instrument such as the piano.
Mastering the illusion of a singing voice cannot be obtained through the shrewdness of not physically lifting a finger until another finger lowers a key.
What is required is instead the ability to precisely adjust the key’s lowering speed, or more precisely, to control the acceleration of that lowering speed.
The art of playing legato is precisely the ability to decrease that acceleration, a result that can be achieved only through the movement’s muscolar control.