THC is stored in fat cells, but when fat cells burn, the THC metabolites are released back into the bloodstream and end up in your bladder as a result of being filtered from the bloodstream by the kidneys.
If a person is still "dirty" (lets say, after taking a home test) and the U/A is 0-3 days away, then all fat-burning activities should be avoided. The person should eat large meals and be lazy as possible during this period to prevent releasing a bunch of THC metabolites into the bloodstream which will end up in the urine to be detected.