To deliberately lose a set or stop trying competitively, usually to conserve energy for the next set or because the current set is a lost cause. Sometimes called 'strategic tanking.' It's technically against the rules (lack of best effort can draw a code violation), but proving intent is nearly impossible. A player hitting every return into the net at 0-5? They might be tanking. Or they might just be having a bad set.
"He's down 1-5 in the second set and suddenly every serve is going in at 80 mph. Is he tanking? His coach looks away."