How does power limiting work with audio amplifiers?

[e100]

A number of modern guitar amplifiers have a power setting switch, for example 0.5w/50w/100w so that they can be used for practicing at low volumes without turning the volume control down to almost zero with consequent loss of fine control.
Is this true power limiting, or is it just an input signal attenuator, as in if you were to massively overload the input, you'll still get 100w coming out the other end?

For true power limiting, I would guess for a class D amp you could reduce the DC voltage feeding the output transistors? Is this what they do, and if so, is there an equivalent for old style class B amps?

[Zero999]

It could be an attenuator or it might alter the gain of the amplifier, by changing the feedback resistor values.

You'd have to look at the schematic to be sure.