How? It has to do with antioxidants -- probably selenium compounds and quercetin -- in the juice reducing the concentration of free radicals in the body in a way that boosts testosterone production.
Of course, the Buzz Killingtons over at reddit are already saying that rats and humans are too different for the effects to be similar. But what do they know??
Find out some more here: http://www.ergo-log.com/onionjuice.html