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.
According to Christine: Slang in Boston for turning a corner; in this case turning left.
also see bang a right
All: Hey Christine where is the living room?
Christine: All Y'all can just bang a left after the set of life size inflatable dolls of the entire Red Sox baseball team.
ALL: (long pause) WHAT?!
Jacob: (just walking in) You have blowup dolls?
Used while giving driving directions. Substitute "left" for the direction that you want to "bang" or turn.
Yo, bang a left! See that light ahead? Bang a left there.