This is a great exercise I have pinched off BE trainer Jo May!
It helps the horse with sharpness in front and making a better shape over the fence.
The fences that looks like a ladder is a bounce of 3 cross poles and the fences at the bottom are oxers with a pole one stride before. To begin with the oxers are jumped out of trot and then built up. Ideally jumping an oxer then the bounce, oxer and then bounce in order to get the horse athletic and dropping its neck. The sharp turn before the oxers engages the hind end and the rider needs to make sure the horse is soft and balanced coming round the turn. The pole on the floor sets you up for the oxer. 
This looks like it might be a bit tight in a 20 x 40 or will it just about work? Our jumping field is rock hard at the moment!
It would probably fit in a 20 x 40 with some careful planning. It was in a 20 x 60 that I did this but there was loads of room at the other end. Hope that helps.
Thanks Lucy. I tried it tonight in our 20×40 and although tight, it worked really well. I kept the heights low, and we just focused on rhythm and balance with our young exracer. Great exercise, thank you.