Graham Hinton wrote:
Normally an amp drives between its output and ground (the black terminal).
If the power rail are +/- V volts the maximum voltage across the speaker is
V volts.
In bridge mode the inputs of both amp channels are connected together *but
one is inverted* so when one output is +V the other is -V and the voltage
between the two red terminals is 2V.
Doubling the voltage quadruples the power = V^2/R.

Which is why they also specify twice the load impedance (R here) to only supply
twice the power so as not to exceed the current rating of the amps.