Booting Smack

(image from Boing Boing)

Having discovered this NYC guide to shooting smack (HT Boing Boing), I was little perplexed at the final guideline: Only "boot" once or twice in one shot.

Honestly, I've never heard the term "boot" in reference to drug use before, but drug users are famously inventive linguists (see here) so I rolled with it (note that this definition of "boot" is only 5th on Urban Dictionary's list of meanings), but I'm not clear what the guideline means. If "to boot" means to "to inject" then what does it mean to inject more than once in one shot? What does "one shot" refer to?  Apparently it does not refer to the injection, otherwise that would mean "only inject once or twice in one injection," which is incoherent.  Since this is buried under Tip #6: Take Care of Your Veins, I wonder if it means "only use a particular vein for one or two injections at a time" where "a time" means multiple injections in a short period or something like that. Urban Dictionary does not list this as a meaning for "shot," but the Drug Slang & Terminology Vault lists "blow a shot" as meaning "when an injection misses a vein." But that doesn't seem to quite have the same meaning either.