Chemaine had us begin at the walk. We started in the corner From A to K where we prepared for a leg yield right. The purpose was to get the haunches leading which is not how you'd typically ride a leg yield. This does two things. First, it gets your horse moving off your outside leg, and it allows you to set up the haunches for the half pass. By getting the haunches to lead, they will ultimately be in the correct position as you change the bend for the half pass.
Once Speedy and I had it mostly figured out, we moved to the trot and started the half pass without doing the leg yield first. To help me set it up correctly - after I had started it as a hot mess, Chemaine reminded me to ride into the corner thinking shoulder in. This put Speedy on the outside rein so that I could then use the rein to send his haunches over. I'll let Chemaine explain it better: