"What is the purpose of the "Flip" switch on Mackie's 8-Bus mixers?"

Mackie's 8 bus is what is known as an in line mixer, which means that you are able to monitor playback through the same channels that you are using for tracking. If the flip switch is in the up position, the main part of the channel (the part with the fader) is monitoring the microphone and line input jacks. If the flip switch is in the down position, the main part of the channel is monitoring the program material in the like numbered tape return. When you are finished tracking on a particular channel, simply engage that channel's Flip switch and the playback off of the multitrack recorder will be routed through the same channel components that usually carry the mic/line input signals. This is a benefit because it allows you to mix using the same mixer utilities you have available when tracking.