Vibrato

Vibrato swings the pitch below the note and back up to it — it never goes above. The note you are playing is the ceiling of the swing, which is why a rolled-back finger still sounds in tune: the ear takes the top of each cycle as the pitch. Switch the vibrato off while a note is sounding and you will hear that the steady tone matches the top, not the middle.

Two things vary. Speed is how many oscillations happen per second — around four is slow, around eight is fast, and everything worth playing is in between. Width is how far below the note each swing reaches, measured in cents (a half step is 100). An extremely wide vibrato on F really does dip all the way to E; most playing lives nearer a fifth of that.