I have been using Muse (no affiliation) which has a headband that measures brain signals, for me, it seems like it is able to tell when my mind has wandered off thinking about life's minutia, when I am supposed to be concentrating on breathing. It gives the feedback by stirring up the sound of waves and wind and really tries to get you back on track. This is my prompt to let go and recommit to focusing on my breathing and then the waves and winds will calm down again.