From the article:
The team found that a compound in red wine, known as quercetin, partially blocked the action of an enzyme called UGT2B17, which looks for testosterone and then sends a message to the kidneys to excrete it.
Professor Naughton stressed that the research had so far been conducted in test tube experiments and had yet to be trialled on humans. “A full clinical study would be needed to determine the effects on people but, if the same results were found, it would confirm that compounds in red wine can reduce the amount of testosterone in urine and give a boost to testosterone levels,” he explained.