With a suitable/appropiate amp normally.
I'll try again. My original post is empty.
Does an AV Amp have one power amp and internal switches send power to the relevant terminals?
No. An AV amplifier or receiver will have a number of separate, independent channels of amplification. For instance, a 7.2 machine will have seven separate amplifier channels, and two line-level subwoofer output signals (each which may be connected to an active subwoofer). Active, as opposed to passive, meaning that the subwoofer has its own internal power amplfier (from, say 100 watts to 2,000 watts) and doesn't have to use power from the AV amplifier. If you buy an all-in-one unit consisting of Blu-Ray player, AV amplifier, five satellite speakers and a subwoofer, then that subwoofer will be passive because the subwoofer will derive its power from the AV amplifier.
You don't have to use all the channels of amplification that are available. For instance, you could have a 7.1 amplifier but use it in only 5.1 mode, leaving two channels of amplification unused.