ROFL - My trainer yelled at me "THAT WAS AN AWESOME LEG YIELD!" the other day, and I had no notion what a leg yield was. All I knew was that I wanted my horse to move forward and closer to the rail, and I did a few things that I felt would make that happen, and they did, and apparently it was a leg yield. :)
So, and I know that this is next-to-useless because there are serious dressage riders here - but until they weigh in, here's what I do (and she's told me that I do this a lot...I just didn't realize it had a Name). If I want my horse to go more to the right (I don't want to use the terms inside and outside because it seems like they mean something different from the regular meaning)...left rein firm against horse's neck to keep shoulder from bulging out. Right rein to the outside for direction (not up, but to the outside). Right leg on a bit, but not hugely, just keeping horse going forward. Left leg back behind the girth just a little, and on firmly to move hindquarters over to the right. So I think (and this is super-tentative, because I'm not training for dressage at this point...although I'm certainly considering it!) the right leg is keeping the horse going forward, and the left leg is giving horse signal to move over to right.
Any minute now, someone who really knows what they're talking about will be along - for me, I thought of keeping my horse straight and just moving him over to the side, and it works beautifully...he's even done this at a trot...and believe me, I did NOT know what the heck it was at the trot - felt like the weirdest movement ever, but it had the effect I wanted which was forward and sideways movement, and my instructor (who does dressage) is stoked about it.
Best of luck!