We've not worked with a flag with him because he hated it, and we didn't want to make things worse, so we just let him be when it came to the flag. I understood it was a problem on Thursday when we couldn't drive him into the trailer with it. His default escape plan is to back up, so when I tried to put pressure on him with the flag, he'd bolt backward.
Jessica had her hands full working him out of reverse; it seems to be his favorite evasion, followed by rearing. He dragged her all over the property going in reverse for a very long time, but she couldn't let up with the flag. We had inadvertently trained him to back up with it because when he'd go bolting backwards, we'd stop, thinking we were doing something wrong. We were - we stopped. If we had stayed with him until we got forward motion - no matter how long it took - we could have probably gotten him into the trailer ourselves on Thursday.
Unfortunately, we reinforced that back was the right answer and she got to undo that training. He's one stubborn redhead, so it took easily a half an hour before the thought to move forward. As soon as he did, she stopped badgering him with the flag. It took a good bit more time before she could reliably get him moving forward off of the flag.
When she did, though, it was time to take him to the trailer, where she completely screwed with his mind.
Eventually, the head wasn't enough, so she started pressuring for the front legs, then the back legs. It took about twenty minutes to get him in the trailer the first time, but after that he pretty much self-loaded. When he loaded and unloaded two or three times, Jessica deemed him trailer trained enough to haul to her place.
It took a while to unload him at her place - he really, really likes the relaxation of the trailer - but he unloaded nicely and followed her up to his new home-away-from-home.
(The video is very long and at times like watching paint dry, but it's cool to see his progress.)