APrior to having his urinalysis, Jim drinks a bottle of vinegar, and then does something to his penis with a turkey baster. However, no explanation is provided as to what he does with the turkey baster, aside from it being "a trick he picked up in the army".After Mike asks Jim why he drank the vinegar, Jim replies that "it shuts down the old kidneys". It's a popular theory among fans that he then uses the turkey baster to pump clean urine through his urethra and into his bladder. Because his kidneys have been shut down, he wouldn't be producing his own urine, hence he could 'hold' the fresh urine until releasing it during the test. A discussion of this technique can be found here (although the discussion is not related to the film in any way).On his commentary track however, David Ayer says that what Jim is doing with the turkey baster is squirting saline solution through his urethra and into his bladder. This will dilute his urine but will remain undetected itself in the test. It is subsequently revealed that traces of THC metabolites were found in his urine during the test, but only in minuscule doses. This is despite the fact that Jim has been taking drugs only the day before the test. The reason for this discrepancy is because his urine has become diluted by the saline.As regards the vinegar, the notion that it can shut down one's kidneys is false; the only known effect of vinegar on the kidneys is that it can help dissolve kidney stones. On the other hand however, there is a belief among some that vinegar can mask THC in the urine, and as such, Jim's drinking of the vinegar is most likely simply another part of his attempt to mask his drug use, just like his injection of saline. The theory claims that because vinegar is a diuretic, meaning it draws water out of the body, it would thus have the effect of diluting ones urine, because it lowers the pH (acidity) of the urine. However, according to Neonjoint.com "There is a myth that drinking vinegar will mask drugs; it won't" (quoted here). A more detailed analysis, which says the same thing, states,
THC is fat soluble, and it gets stored in fat cells. Cleaning it out of the lipid tissue is very difficult. Many products claim to clean out the system, yet they do nothing to remove THC by-products from fat cells. A study was done in Germany in 1993 on 50 of the most common herbs used by people trying to pass the test. All 50 herbs failed to cause a negative. Unfortunately, this rumor will not die. Goldenseal is useless; yet it's the most common thing for people to use. The only way to extract THC from fat cells is to exercise. Fat cells secrete fat with THC metabolites at a constant rate, regardless of what herbs you consume. You may be able to temporarily clean THC metabolites from your bloodstream, or dilute your fluids to yield a larger urine/THC ratio, but your bloodstream will continue collecting THC metabolites from fat, and your urine will continue collecting THC metabolites from your bloodstream (quoted here).
Whatever the case about the validity of the technique, Jim's drinking of the vinegar and injection of saline are both means to the same end - to mask the drugs in his system by way of diluting the toxicity of his urine.
AFor a complete breakdown of the weaponry used in the film, including numerous images, see the IMFDb entry here.
