In essence, this exercise asks that you ‘push’ forward on the long side of the arena and ‘collect’ on the short side of the arena i.e. on the long side of the arena, you extend the gait and on the short side, you collect it.
This exercise is excellent for developing strength, balance, and reactivity to the rider’s aids. However, horses can start to anticipate the ask of this exercise quite quickly so it is always a good idea to avoid ‘drilling’ the exercise and move on to something else once your horse has got the idea.
NOTE: It is essential to gauge the level to which your horse is capable of collecting. You want to slow the pace but maintain the energy and not have them break into a different gait. Similarly, you don’t want to push your horse too much down the long side as this risks a total loss of control.
Like with all these exercises, build it up slowly; it doesn’t need to be perfect immediately – there is always another day.