sandbagging is widely used in sports where it may be advantageous to hide your true skills until an appropriate moment, such as when a wager has been raised -- I've heard it used in bowling and golf and billiards. the betting aspect is actually a clue to its gaming origin, which is poker:
The specific "hang back" or "slack off" sense of "sandbag" you're wondering about comes from poker, where it originally described a player who held off raising the stakes in order to lull the other players into a false sense of security. The poker sandbagger would pounce late in the game, clobbering the other players with his good hand. More generally, "sandbag" has come to mean to under perform any task in order to gain some advantage.
-The Word Detective
the reference to "clobbering" gets at the ultimate origin, which is the use of a sock filled with sand to strike someone, the object of this being to inflict pain without leaving a mark.
