Well, if sandbagging refers to the clobbering with a sock filled with sand, this would be analogous to the final phase of the procedure and not to the preliminary phase of allaying suspicions. In actual use I have always interpreted the phrase to mean the laying back in the early stages; the racer who barely qualifies and then blows everybody away in the race itself is said to have been sandbagging in the prelims.

I was nonplussed by the assertion that the phrase originated in poker; I was not and am still not aware of socks filled with sand being standard equipment in poker games. Perhaps I've never played with sufficiently serious players; my poker experience is not extensive.