In Boston's eight wins in their last 10 games, the Celtics have been sharp on defense, sporting a 94.7 defensive rating. They've scored over 110 points in seven of those W's (103 vs. Indiana), but it's really been their D that has been the source of their success and furthermore, given this team their identity.
The modern NBA is played on the perimeter and with superior defenders like Avery Bradley, Marcus Smart, and Jae Crowder, the Celtics smother opposing teams outside the paint. Especially when they go small, they can switch every pick and take away an opposing team's first and second option.