The announcer just said the Boston power play is 5 for the last 105. hmmm that's about 4.8% success rate.
Now at regular strength the Bruins are getting around 3 goals a game. Now 3 / 60 is 5%. But it's actually higher as they aren't even strength the full 60 minutes.
My point is the Bruins score more frequently at regular strength than they do on the power play! So mathematically their strategy becomes obvious. Just do the exact same thing on the PP as you do at regular strength. Don't put on a PP unit. Use your regular lines, even the 3rd and 4th lines. Don't run set plays or attempt anything you wouldn't do at regular strength.
So even if you do nothing at all different Boston would still score more often on PP then regular strength because with only 4 defenders over time they will inevitably score more often.