According to Savaii memeber Jim Rice, the tribe had the puzzle figured out even before the challenge began during episode 2 this past week. Rice used Twitter to claim "We had that puzzle figured out before Probst said go, knew what to do, 0 hesitation, 0 wrong moves." Apparently, during the portion when Probst instructs the tribes on how the challenge works, Savaii whispered to each other the solution and so they were able to come back against Upolo after being behind early.
My View: Rice is a blowhard so its hard to verify this but the way Savaii came back so quick with no wrong moves on the puzzle lends credence to this. Either way it was an impressive performance by them in that contest.