There was an interesting discussion
here concerning tailplane lift (downforce) and stability.
Should answer most questions about this.
Basically, there is a couple between lift and c of g, with c of g being forward of the c of p. The down-force provided by the tail balances the couple. This provides longitudinal stability. As the airspeed changes, the c of p (for practical purposes) moves only very little* vs the amount of lift - or negative lift - provided by the tailplane, which (like the wing) increases/decreases as a function of the square of the speed (among other things).
What that means is that airspeed is reduced, the amount of negative lift provided by the tail decreases, and the nose will try and pitch down. (C of g doesn't change, so can assumed to be constant.) Vice versa if the nose drops, or is pushed down, and the airspeed increases.
You've heard of phugoid oscillations? The more longitudinal stability the more these will be self- regulating; the aircraft returning to its' trimmed airspeed within only a few oscillations.
When the c of g is at the aft limit, longitudinal stability is reduced, but still present. If c of g is sufficiently aft of the aft limit, a situation arises where there is no longitudinal stability, and the aircraft, left uncorrected will, basically, crash.
I've read a flying report written by someone who flew a Baron severely aft loaded. It was an extreme handful. He calculated that the c of g was way outside the aft limit afterwards. I forget the figure, but it was something around 12 or more inches.
* The C of P moves forward on the main-plane as angle of attack is
increased, to the point of the stall, when it moves aft. The value is of the order of inches. This also tends to be pro-longitudinal stability. It's a significant value, but still quite minor in comparison to the changing down-force produced by the tailplane in a correctly loaded aircraft. The tail has a large moment to work through, being way back there, so even if the actual lift/negative lift value is small, it's effective.