The vibrato cycle consists of action and reaction and this 'Knocking the peg' exercise helps develop the necessary rebound.
The most common vibrato action involves the finger rolling down the string (flattening the pitch) and rebounding back up again to the note.
Listen for the evenness and tone of the knock. We can of course vary the width and speed of the vibrato but each style of vibrato needs a constant speed of cycle covering a consistent distance....
