a nitpicker inquires/enquires: why isn't sandbagging used to refer to the process of playing up to capabilities in the latter phase rather than playing under them in the earlier?

Am I to understand that you don't play poker then? Sandbagging in poker is: not playing up to the capabilities of your hand! Let's suppose that you're playing 7-card stud, in which you receive your first three cards en masse, two face down and 1 up, and then the betting ensues. Let's further suppose that your three cards are Kings (don't want to get too carried away here). You certainly wouldn't want to lose your poker face here by showing your true excitement, but you also (usually) wouldn't want to bet your hand to its full worth by raising the limit, as everyone with Q-10-6 will immediately drop out, thus robbing you of some of your just rewards. So you sandbag, merely matching someone else's bet (there's probably an Ace showing) or making some minimal bet whilst opining, "Hey, are we here to play poker or just to drink beer?!"