I think you're talking about Goldenseal, an herbal derivative known for it's cleansing properties.. Here's a pretty good explanation:

Does Goldenseal work?

Goldenseal does nothing to remove marijuana from the body. People that used Goldenseal and passed the drug test passed because of the extraordinary amount of water that is consumed with Goldenseal. Excessive fluid intake can result in the reduction of creatinine levels in the urine and result in the sample being considered too dilute.

http://www.bdtzone.com/false_positives.asp