Yeah, its not obvious is it?
But QFF is more like QNH than QFE. QFE gives zero on the ground, whereas QNH/QFF both give an amsl elevation reading.
It's a bit subtle, but as I understand it you take your pressure at your location. For QNH you then increase it by assuming ICAO standard atmosphere conditions between you and sea level (imagine standing on a clifftop). For QFF you do the same, but apply the actual temperature (as opposed to ICAO) to the imaginary column of air beneath you.
Hmm... hard to express. How did I do?