Slang. Typically heard in Boston. It is not just making a left turn, it is to make a left as soon as the light goes green and before the oncoming traffic has a chance to react to the light changing to green. If you ever find yourself driving in Boston it has almost become customary for you to allow the first car in the turn lane to “bang a left” in front of you before you pull forward from the light.
I can't believe that there's no green arrow at this busy intersection, it looks like I'll have to bang a left if I ever want to get through it.