Taking a
left turn immediately after the light changes to green.
Boston area roads tend to have one lane in each direction; if the first driver at a light needs to make a left turn, frequently only one or two cars would be able to get through the light on each cycle.
Many assertive drivers will extend the principle of the Boston left and attempt to forcibly take a left turn when the reach the intersection; they do this by slowly inching out until someone yields right-of-way.