"Use martelé to practise your tapping fingers. Make sure your finger goes down before the bow. Also use pizzicato".
Notice how the fingers go down rhythmically halfway through each beat.
Both pizzicato and martelé have a clear articulated start to the note, followed by a release. This makes them ideal for making sure the finger is ready to play. The fast drop of the finger is a tapping action which gives correct weight and contact on the string.
When playing descending passages the finger should lift with similar energy and speed to its tap-down....