kinda surprised no-one else has chimmed in.
this is what ive always thought it was anyway. anyone else is more than welcome to chime in. and tell me its not correct lol.
using multiple amps with direct out's for your tones of lead, clean, crunch, solo, etc. independently running from each other into a single poweramp and cabinet setup.
many ways of doing it, easiest is a bob bradshaw racked isolation switch made for switching multiple heads between each other with a load on the OT for a single cabinet. could use a load box instead and use the amplifier's line out, or, if the amplifier doesnt have a line out you could use a load box as a line out and still work that way.
at least, thats what ive always assumed it was.
-matt