It's most likely a vasodilator of some sort - it opens up the blood vessels, more blood flow means more heat gets released from the body. The reason fescue causes problems is that the endophyte is a vasoconstrictor  - it narrows the blood vessels so the cows can't get rid of the heat as well.

Tasco, Endo-Fighter, and Heat from VitaFirm are all vasodilators. Of the 3, I'd have the most confidence in Tasco.

I'd love to see the "research" that shows a 30% improvement in weaning weight - that would be downright miraculous. Since the calf starts at ~80# and not 0# at birth, that means the calf's daily gain from birth to weaning has to increase 35-40%, and since converting milk to calf gain is not 100% efficient that means the cow would have to produce 50-60% more milk per day.