Fenugreek is a herb that has been traditionally used for diabetes and for male virility (such an interesting word, virility), you can read more up about it on its page.
The major update comes from one study, it is the human study currently excluded from the Human Trials table that shows a decrease in DHT.
For a quick overview, testosterone converts into DHT (dihydrotestosterone) via the 5-alpha reductase enzyme, and this enzyme is the only pathway for the conversion. Inhibiting this enzyme is seen as a good thing for androgen responsive prostate cancer and for androgenic hair loss, since it reduces overall androgenicity in the body.
DHT and testosterone both act on the cell to induce 'testosterone like effects', or androgenicity. DHT is more powerful, and can be seen as 'super testosterone'. This is why inhibiting DHT production lessens the overall effects of androgens, and benefits the adverse side-effects of androgens (as mentioned prior, prostate cancer risk and hair loss). Other stuff like Ganoderma Lucidum or Soy isoflavones may also reduce the risk of prostate cancer via this mechanism.
However, this also applies to muscle building via androgens.
Many people use fenugreek to 'boost testosterone', which it technically does and might be merely because of a backlog of testosterone. However, by inhibiting DHT it can reduce the overall effects of androgens despite the increase in testosterone.
This may mean that fenugreek supplementation for muscle building is either inert or counterproductive (I allude to the initial study finding testosterone increasing effects, that noted no differences in lean mass or power output); at best, it will just not be a good androgenic muscle builder despite being one of the few herbs to increase testosterone in humans.
Basically, fenugreek just took a hit to the efficacy when looking at how good of a muscle builder it is