No. It typically reduces the power supply voltages that feed the output stages of the amp by use of a power transformer with two secondary windings. It essentially transforms the amplifier into a lesser powered one. An "ideal" amplifier that is capable of outputting 200 watts into 8 ohms will output 400 watts into 4 ohms. Many older stereo amps came close to that ideal. The Altec solid state professional amps that I have will do that easily, and they will do almost 800 watts into 2 ohms.

I'm not singling out any particular manufacturer of home theater amps - almost all of them make use of this switch.