It is specifically mentioned in the HST FAQ. You can find it by going to the HST FAQ forum under Training. Click on the Supplements topic title. It is mentioned in the second post and further down. I suggest you read the whole thing. While you are poking around go ahead and read everything you can in that forum. Lots of good info.
Personally, I have never used it... not that I wouldnt. I just havent got around to trying it out yet.