The accompanying foot pedal to the 5150 amp. Very simple:

-the left switch (channel) changes between the rhythm and lead channels (lead is a tad louder).
-the right switch (effects) controls the effects loop* on/off status.

* OK, here's a breief description of some points...

The 5150 head has a preamp and a power amp. Your sound is made in the preamp, and sent to the power amp, which gives it enough juice to power the sound through the speakers.

The effects loop sends the preamp signal out to effects, etc. hooked up in the loop, then returns (gets) the effected signal back to the power amp. Look at it as if you wrote some lyrics, gave them to me, I added this and that, then gave it back to you, and you published the final product. That's the basis of an effects loop.

Get it? Got it? Good. :-)