Well, handily enough I did leg yielding with Darcy today ^^ With Charlie, you can kind of go on autopilot, but with Darcy you have to be much more aware of what you're doing. So knowing all this is really helpful because it stops me from accidentally telling him to do something other than what I want!
I only had half of the large outdoor arena, so I was leg yielding from the 3/4 line to the wall heading down the long side, which was 30m. At first he was pretending not to get the idea, but once I reinforced my leg with my seat he was zipping across! So I guess that answers the 'does Darcy listen to your bum' question.
I paid attention to his shoulders and they weren't bulging in either direction, and my instructor said the turns were fine. :) It's weird looking down; I don't normally do it at all apart from a glance for my diagonal. My seat was also much better than the last time she saw me ride (I haven't had a lesson with her in a few weeks) and my lower leg is better and aids are clearer. I don't confuse Darcy by, say, asking for a turn in one direction and squeezing with my leg to make him go the other way.
So all the info you've all given me is really helping out! Thank you! And I'm glad it's interesting for you too, Kirsti :)