its the same as every other amp out there, the company is just cheap.
voltage is based upon differences, kinda like height/gravity -- a man at 1,000m above sea level can fall 2m and live, but a man at 100m above sea level cannot fall 100m and live -- the absolute height doesn't matter, only the distance of the fall. same with voltage. you can have 2 10,000V lines and short them together with no consequences -- they are both at 10,000V so the voltage across the short is 0V.
in a normal "bridgeable" you have the following connections:
Ch1 + = +V out
Ch1 - = 0
Ch2 + = 0
Ch2 - = -V out.
because (+V - 0) = (0 - -V) the amp works as a stereo amp just fine. and you can set up a bridged mode by hooking across (+V - -V) = 2V out. (basically you can double output voltage and thus double output current, which means 4x power, but each amp is putting out its normal voltage with twice the current. this means R = V/I, and since current has doubled, each amp channel acts as if it were driving a speaker of half the impedance of the actual speaker)
Because it is confusing to users to attach ANY wire labled as + directly to a wire labled -, and because users perfer to have an equal number of terminals and wires, this arrangement is not used in most amps.
it is perfectly valid and gives no performance advantage or disadvantage
if you use a DMM on most amplifiers, i'd bet you'd find pairs of output terminals are indeed shorted together.
