This may not seem a very exciting exercise, but it’s very good for the horse, getting it to wait, listen, and respond immediately, and also for improving the rider’s reactions (because you have to keep completely changing your aids.)
It is quite difficult to do spot-on!
You simply zig-zag in and out, to and from the track, changing bend at the end of every part of the movement.
Young horses usually find it easy to step across towards the perimeter boards/fence but aren’t always so willing to step away from it at first.
I often count how many zigs and zags I can do before reaching the other end, so that I gradually increase the difficulty.