The answer is here.
http://www.frsky-rc.com/ShowNews.asp?id=43HS mode is for Digital servo's, do not operate analogue servo's in HS mode or you risk damaging them.
This was introduced to fix the jitter some times experienced on the Digital servo's.
Right at the bottom of this page....
http://www.frsky-rc.com/ShowProducts.asp?id=39There is this warning :
"Warning: HS Mode is only applied for high-speed digital servos. Other servos should select FS Mode, otherwise servos will get heat or even burn out."
There is also some confusion around the naming of the rx's. As far as I am aware there are only TFR8's just checked the latest pricelist and indeed there are no TFR7's.
There is supposedly a difference between the HK (orange branded) ones that they can only do FS mode and not HS mode. This may have changed since my last chat with FrSky however.
I will give them a shout tonight when I get home and try get some clarity on this.