What are send and return levels for in an effects loop?

[tjinca1]

Just got a PRS 50 watt combo (the H version), and in the back effects send and return at has a level knob for the send and the return...

How do the effects send and return level knobs work? Where is a good setting? Half way? Full bore? Does it lessen the gain as you turn it down, or is it strictly volume? And what are they for? I've never had an amp with them. They don't seem to just change effects plugged into the send and return, so what do they do?

[SteveO]

The effects send determines the signal level going to whatever effects are in the loop, and the return sets the level coming back into the amp. It's most common to run the send about halfway (or wherever your effects are happiest without distorting), and the return is usually used to set the level coming from your effects so that there is no increase or decrease in volume when you toggle the loop on and off.