Blu, the so-called "bi-amping" feature that some receivers advertise has been discussed here several times in the past year. It isn't bi-amping and should be disregarded since it can't double the power available to a speaker, as some imagine, or increase it in any amount. The unused back channels have no power of their own to lend; they're simply acting as valves in metering out the same amount of power from the receiver's one main power supply section.
Bridging is entirely different and is a valid procedure, although it might be unnecessary if the regular connections already supply more maximum power capacity than needed, as is typically the case. This works by inverting the voltage that one channel outputs and then sending both it and the normal voltage from another channel(both channels being fed the same audio signals)into the positive and negative speaker terminals. In effect the positive voltage "pushes" while the inverted negative voltage "pulls" in the same direction, co-operating in driving the speaker cone rather than cancelling each other in the driver, which would happen if both were of the same polarity.
The double voltage input theoretically would increase power four times(not double)since in one form of Ohm's Law power is proportionate to the square of the voltage(P=E[squared]/R). In practice the process doesn't work with theoretical efficiency and the typical power increase is no more than about three times.
One possible difficulty that might arise is that with the receiver having to put double the voltage into a speaker, the current would also have to nominally double(Ohm's Law again, I=E/R)with the speaker impedance of course remaining constant(the receiver doesn't "see" half the impedance, as is sometimes written). This would increase the possibility of overheating and protective shut-down.
