What is a bridged amplifier?

“Sugden have been making Bridged amplifiers for forty years and it is a way of doubling power output by using a stereo amplifier as a single channel mono amplifier. 

It is important the stereo power amplifier has a large power supply with plenty of current capable of driving a low impedance. The signal to half of the stereo amplifier also requires inverting so a normal stereo amplifier cannot be used without additional circuitry. The Sapphire FBA-800 is a bridged design and uses four pure class A mono amplifiers in one chassis.”