Amplifier bridging is simply using 2 channels of an amplifier to drive a common load. For 2 channel amplifiers, one left signal and one right signal is used to drive a mono speaker load. Keep in mind that mono and bridging are not necessarily the same. Mono means that there’s only one output signal. There could be more than one speaker but each speaker will have the same output. Bridging means that you are using more than one source of power to drive a load (speaker). The sources of power are one each output from either channel of the amplifier. A long time ago, amplifiers had signal on the positive output speaker terminals only. To bridge one of those amplifiers, you’d have to use some means to invert the signal on one channel (remember the old ‘bridging modules’ for Orion amplifiers?). Today’s bridgeable amplifiers have an inverted channel as part of their design. For many amplifiers, the left positive and right negative are are the signal outputs. A few use the left negative and the right positive. Others still (mostly mono amplifiers that are to be used in bridged pairs) require that you choose 0° or 180° via a switch to invert the signal.