This is one of my favourite dressage exercises. You can try it in walk, trot and progress to canter. Sometimes I'll go straight on the diagonal from P to I, sometimes I'll leg yeild. Then as you cross the centre line on the 10m circle, you can do a downwards transition (trot-walk-trot or a simple change, in canter). I think it's great for keeping the horse engaged and fine tuning the riders aids, as they have to think fast and be well prepared to get balanced transitions.