I *think* this has been discussed before, but "cg" and "aft" are both under the four letter search threshold, so not easy to find the thread(s).
From what I recall, it's actually the use of a Forward CG limit which is moved aft of the 'normal' one. That than allows the stall speeds etc to be calculated for that "less forward" limit, which will give slightly lower stall speeds (due exactly to the tail lift/downforce issue mentioned above).
If my recollection is correct, it should really be called an alternative
forward cg option, because
AFAIK the aft limit itself isn't moved, and is in amy case pretty much irrelevant* for most performance purposes.
* unless you start mucking about with Vmca etc., but I can't see anyone going down that route ....