rubato explained
The expressive flexibility of tempo in which some beats are slightly lengthened and others shortened, while the underlying pulse remains steady
In Depth
True rubato, as understood in the Romantic era, involves a push-and-pull of time: the melody lingers on expressive notes and hurries through others while the accompaniment maintains a steady beat underneath. This creates a sense of the melody floating above the rhythm, giving the music a spontaneous, speech-like quality. Chopin was the supreme master of rubato, and his student Mikuli described it as a tree whose branches sway in the wind while the trunk stays firm.
Chopin reportedly placed a metronome on his piano during lessons to ensure that his students' left hands kept steady time while the right hand played with rubato.