Laws of Blues Harmonica - Accompaniment Rule of Inactivity

When the vocals begin, hold the last note you you were playing softly until the vocal phrase is over and then continue playing (this assumes you were playing a chord tone, otherwise you would of had to change quickly to a chord tone). If you were playing a riff (repetitive lick), keep playing that riff softly through the vocal phrase. Holding a note equals inactivity and continuing an already-established phrase (riff) is inactivity. Change is activity and will distract from what's happening with the vocals.