Out of all the studies done, the only conclusive evidence is that it is a "pro-erectile". (I swear that was the term used in the study)
There is no actual proof to date that it increases free testosterone levels in any amount; contrary to what supplement companies say, plant sterols DO NOT increase free testosterone levels.
On a personal note, I did notice increased libido (as do many), but offhand I don't know the mechanism.
I did have my free testosterone levels checked by my family physician when i was on Trib, and there was no difference from when I wasn't on it.