OK, so now that I've finally gotten all that theory and technicality stuff down, I can finally update on our progress today!
I went out with the goal to just work on what we started yesterday (backing up with a verbal cue), adding speed and getting him to respond to the verbal cue. AND, per PunksTank's suggestion, I wanted to make sure I kept our training session short.
I was pleasantly surprised with how yesterday's lesson apparently "sunk in" overnight! I'm betting that the same thing would have happened even with just a short break yesterday like PunksTanks suggested. Unfortunately, I board my horses so it's a little difficult to spread out our sessions with breaks, but I'll have to get creative. For now, I'll just do little mini-lessons. I'm not sure how long I was out there today, but I made a point of stopping while we were ahead and keeping it shorter than yesterday.
I turned Flash out in the arena as soon as we got out there. He was eager to find out what game we were playing today, so he followed me wherever I went and stopped respectfully when I did (we've worked on where he's supposed to walk respectfully before and he got a reminder the other day when I reacted by shaking his halter without the clicker - he's been very respectful since). Then, I turned around and said "back up" - and he took a step backward! I immediately clicked and treated. He's backing up about 50% of the time on just the verbal cue now and will continue backing up if I keep saying it (backupbackupbackup...). He'll even do so at a decent speed, through I still want to get him faster. If I pick up my energy and walk toward him, shaking my finger at his chest like I did yesterday, then he picks up speed and moves pretty well.
I forgot to mention yesterday how he was swinging his hip some and not backing up straight, but I fixed that by swinging the lead rope at his hip and turning his head slightly, so he straightened back out. He seems to have worked the "straight" thing out now, especially since we're picking up speed and he has to move fairly straight in order to do so quickly.
After a few minutes of backing, he wandered off. I think he's feeding off some other cue he's not quite understanding and that I'm not trying to give, because he basically lunged himself on his own for a while. That alerted me to the fact that I needed to teach him a "come here" cue since he was so convinced he was supposed to be going around me (I try to do most of our training at liberty and didn't have the lead on to stop him). So, for the next few minutes, I focused on just asking him to come. Essentially, I called his name and extended the back of my hand to him and had him target it. Pretty soon, I could send him off by swinging the lead rope and then ask him to come in and touch my hand. Once I had his attention again, I asked him to back up a few steps, then come back forward when I called him and extended my hand. We only did this a few times, and then I decided it was a good place to stop while he was still interested and paying attention.
Tomorrow, I think I'll continue working on the "back up" and "come here cues" and focus on those until we have them really well :)
One last interesting note - we worked a LOT on leading last year out of necessity, including trotting when asked. He knows his cue very well, even after he had the winter off, and immediately trotted up to me when I asked him to catch up while leading him to the arena. However, he never passed me and slowed down as soon as his head was at my shoulder. It's so nice to have a cutie trotting after me and managing the slack in the lead rope appropriately!