I was told this one last week and have used it a few times since, and it’s a useful one.
The poles are just 2′ apart, and you approach in walk (ridden, although it can of course be done in hand).
With a really forward-thinking horse this might need to be a collected walk, otherwise they try to step over two at once!
An educated horse will probably slow down and consider the ‘puzzle’ as you walk towards it, a clueless youngster might just march in and get a bit of a surprise when it realises how close together the poles are!
The close spacing means that the horse really has to engage its core, and think about where it is putting its feet.
Once they get the hang of it, you can raise alternate ends of the poles, or even the same end all the way if you want to strengthen a particular side.
I was told that it’s better to raise the side that the horse is stronger on, so that it has to push more with the weaker side to lift the stronger side up. That sounds weird but makes sense. So, my mare is weaker in her right hind, so I’d raise the left side of the poles.
Pound-shop potties make great mini-blocks to raise poles on!