thorn,

That depends on your radio. If it is a computer model the mix is what you assign. I had an old futaba that had fix mixes and there may be some of this type still out there. The mix switch will 'mix' your input signal into multiple outputs. Let's say it is a rudder-aileron mix. If you input aileron, you will get coordinated rudder servo action as well, without moving the rudder stick. You can have v-tail, flapperon, elevon mixing, to name a few of the basics. If you have a typical 3 flight control airframe, mixing is not needed.

Have fun!

Bedford