My understanding was that chasteberry stimulates natural progesterone levels slightly, which may have an anti-estrogen as well as anti-prolactin effect in men. Prolactin is a hormone that in men, is kinda like estrogen, cortisol, insulin, etc, in that it tends to rise with age, suppressing hormones like gh, T, etc.
Bromocriptine is an anti-prolactin it works by enhancing the sensitivity to dopamine (essentiually turning up the volume of dopamine at the receptor level), this then works to suppress prolactin via the neurotransmitter influence. Some people have to be careful with bromo as its dose response curve is quite steep, so it is easy to overdose.