in essence the 4 ohm load presents a 2 ohm load to each amplifier, because the center of the load is a "virtual" ground, even if it's not connected to ground (it can't be anyway, because it's the center of the voice coil). if you could connect it to ground, there would be no net current to ground, since there's no voltage there. let's assume that the output of amp 1 is at +80V and amp 2 is at -80V and we have a 16 ohm load (or for a better illustration two 8 ohm speakers in series) the net voltage is 160V and the current is 10A. the center connection of the spealers is NOT connected to ground. so 10A*8 ohms=80V across each speaker, and since the output of both amps is 80V of opposite polarities, the voltage at the connection between the speakers is 0V. the power can be calculated in two ways. each speaker has 80V across it at 10A for a wattage of 800W per speaker(x2=1600W). the total voltage is 160V, and current is 10A, equalling a wattage of 1600W. if we connect only one of the 8 ohm speakers across the amps, the current doubles to 20A, with the same total voltage of 160V, giving a power of 3200W. the load EACH amp sees is 80V/20A=4 ohms.
