Power amplifier bridging principle: 

    power amplifier bridging is to push the output terminals of two amplified lines to a group of loads (speakers or speaker systems) at the same time. Increase the audio undistorted output voltage value of the power amplifier, thereby increasing the power. When bridging, the audio waveforms output by the two amplified lines are reversed and loaded on the speaker at the same time, so that the load has twice the audio voltage, thereby increasing the power by four times (under the condition that the ideal current is sufficient and the load impedance is consistent But in fact, due to power limitation, the current cannot keep up, so the bridge connection is often not suitable for impedances lower than 8 ohms, so the output power is usually doubled.)

Bridge Video Demo: