The simple ones always seem the most effective.
This exercise uses three or four poles on a 20m circle.
Place the poles on part of the circle, at the same distance you would place them on a straight line.
You want to make sure that the horse is bent around your inside leg correctly and that you stay central as you go through the poles.
It will help the horse to engage his hind leg more effectively.
Raised trotting poles is another way of encouraging hind end activity. To add an extra bit of difficulty I place a trotting pole at a ninety degree angle to the first raised pole, you want three or four in a line after that. Trot over the single pole and then ask the horse to bend onto the raised poles.
Both exercises will really help to develop the hind end and encourage more of an athletic and energetic hind end action