The specific "hang back" or "slack off" sense of "sandbag" ... comes from poker

Well, OK. It comes from poker. This would be because the player hides behind a pile of sandbags whilst doing it? This would seem to be the very sort of dead giveaway that having a poker face is supposed to hide. Or is sandbagging used metaphorically to describe the act of clobbering the opponent, as though with a sandbag, when the mask of poor playing is stripped away? And if so, 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? Inquiring nitpickers need to know.