Okay so this scene really gets me because it describes the whole unholy relationship in a few moments…
At first Brittany and Santana were hanging onto each other ( in high school) Santana with no loyalty to Quinn, but Quinn hanging on, hand on top of Brittany’s creating some unity between all three of them.
Then after the break up Brittany and Santana split up and Santana looks back to Quinn.
As Brittany walks away from Santana (dating Sam and going to University) for a brief time Santana is lured in by Quinn (they made it a two time thing.) Brittany turns around again Santana spinning back to her and Quinn walks away from both.