OK AFR = Air Fuel Ratio (http://en.wikipedia.org/wiki/Air-fuel_ratio)
Well when you run higher boost, you obviously need more fuel, right.
But if you run too much boost you will blow your motor, wether you have enough fuel or not, you'll end up with a hole in your pistons or worse.......
So if you run high boost with a stock ECU (computer) it see's the amount of air flow through the AFM (Air Flow Meter) and so when it gets to a certain point, it basically goes into R&R mode, (Rich & Retard) where it puts as much fuel in and retards the timing back to save the motor.
What something like a SAFC does is intercept the signal going from the AFM to the ECU and tricks the ECU into thinking there is less air flow then there actualy is.
It probably does a whole bunch more stuff, but that is about the limit of my knowledge
Please anyone correct me if I am wrong, ie SK