“Rabbit practice” is my name for it. I’m not surprised you couldn’t find anything! 
Before you go any further I’m warning you that this is not a step by step “do X” then “do Y” kind of training. I had to watch the videos (and others along the same lines) and think about my horse’s behaviour and how I might induce a little anxiety and how I could create conditions where my horse would let go of his rabbits. I’m happy to share more details about what I did and help you with how you could apply the concepts to your situation.
Start by watching these videos, or read my post and then watch. At least watch the first video before reading my post. The rabbit theory explains why there are good days and bad days - it depends on how many rabbits there are on any given day.
Theory of rabbits (5 min)
Cognitive Behavioral Therapy for horses (20 min)
I did much of my initial rabbit practice on the longe. The goal was to have my horse volunteer to stand near the scary spot (wherever and whatever form that might take on any given day). Rabbit practice looked something like this:
A quick WTH longe to check attention and response to commands done in a non reactive part of the ring/arena. The purpose was to get his mind into work mode.
Then started a pattern that would take us near the scary space. I liked the cloverleaf pattern for this as it breaks up the circles with straight lines and it’s easy to do lots of transitions (trot the circle, walk the straight or vice versa) and it provides multiple angles and distances approaching the scary space, passing and turning away. Do you know the cloverleaf pattern?
Watching my horse carefully I asked him to shift down one gait as he approached the scary space, T-W, W-H, at the point where he began to show worry about the SS. If he couldn’t, the W or H might happen after he passed the SS and that was okay! This is not about you must halt NOW if I say whoa! It’s about helping him learn to let go of his rabbits and take responsibility for not spooking. After the downshift happened he got praised and we continued to the next leaf.
On each leaf there is a point where the horse is approaching the SS. If he showed concern I would ask for the W or H. On the leaf furthest from the SS that transition would happen pretty much where I asked. Any H that occurred where/when I asked meant letting him stand still for a minute (yes, 60+ seconds) or until he couldn’t stand still because anxiety about the SS was getting too much. Any H or W that occurred past the SS was met with praise and immediately back to work.
The cloverleaf (or whatever) must be work. Lots of transitions - and the ones not affected by the approach to the SS must be quicker and demonstrate attention and response to my commands. This is critical because I am creating a rest space for him beside the SS.
As we worked the transition occurred closer and closer to the SS. When the approach was in T he started to volunteer the W. After he volunteered the walk several times and could W past without any big deviation in track or speed I very calmly asked him to T beside the SS. It sounds like the wrong thing, like I’m telling him to run away, but I’m actually saying listen to me and do this thing at the moment the SS looms largest in his focus. As with the initial downshift it’s not about NOW, but about getting past the focus on the SS and listening to me.
Then we progressed to H beside the SS. At first H happened well past the SS, so praise and immediately back to work. Then H happened sooner. When he H next to the SS I let him stand until he couldn’t and asked him to walk on again before he moved on his own. This was only a few seconds at first.
Can you see what happens next?
The first time my horse volunteered to halt at the closest point to the SS of the day and cock a hind foot and drop his head!! I knew we had cracked it (angels singing 
). And that day’s SS was legitimately scary (odd noises and a narrow slice of visual input).
We did similar rabbit practice under saddle. We graduated to working past the SS without the downshift. When approaching and passing the SS I did as little as possible (quiet legs and hand) then asked him for something as soon as we passed it. As he got better about keeping his track and speed past the SS despite his anxiety with no aids from me, I pushed my hands forward as we approached and let him take responsibility for maintaining track and speed. If he did drop to W that was okay, and I quietly asked for trot again. Oddly enough looping the reins often made him keep the track better.
https://youtu.be/zG5DhgEbiDk Now this figure eight at the SS of the day is usually enough rabbit practice to remind my horse that he can let go of his rabbits. I should note that I did try this technique a year or so before starting the rabbit practice and it did not work for my horse at that time. He needed the CBT to learn to let go of his rabbits.
It took time and lots of rabbit practice, and rabbit practice on days I would have preferred to do dressage. It was worth the effort.
The normal “ignore it and ride like it’s not there” sort of technique can result in a horse capable of handling more rabbits, ie. hiding his anxiety. It’s still there and there is a limit to how much anxiety the horse can contain before it affects performance. I remember an article that used heart monitors to measure stress response in horses facing scary objects/situations (there were a number - one I do remember had the horses ridden at the walk between smoke pots) and the very dramatic, balking, backing, spinning, shaking head, etc horse had a lower heart rate than the apparently calm horse who carefully walked through the smoke. If we can help the horse learn to let go of anxiety it seems a better option to me.
