Brad Stevens really hates size mismatches. Terry Rozier is the smallest player on the floor for the Celtics, and Brad has found ways to counter defensive switches where Terry gets caught guarding a much bigger opponent - which in this series often means Lebron James.
The brief sequence in the tweet below really puts the Celtics masterful switching defense on display. To no one's surprise, Al Horford directs the entire process.
Cleveland seems to be attempting a screen that that will shed Semi Ojeleye off Lebron James and force Terry Rozier to pick The King up.