This exercise was first introduced to me at dressage camp, but I’ve used it so many times since.
Without using the track, scribe a square in the arena in walk. Perhaps with 15m long sides, but the length is irrelevant really.
At the corners, you want to ride a turn around the forehand. The aim of this is to increase the flexibility of the inside hind leg because it is brought forwards and under the horse’s body. This means that the leg becomes stronger and so increases the impulsion from the hindquarters. Turning around the forehand also focuses the rider on controlling the outside shoulder because as the inside hind leg steps under, many horses will avoid taking the weight onto the inside hind by falling out through the outside shoulder. Whilst riding this movement you, as a rider, will also be able to feel the horse bending through their rib cage, which also improves their suppleness.
Once you can ride turn around the forehand easily in the corners, add in step two. Ride shoulder in on each side of the square. Again, this aims to improve the flexibility and strength of the inside hind leg, encourage the horse to take the weight of their body onto their hindquarters and to lighten the forehand. It increases the suppleness through the rib cage.
Initially, you can use a big square, and use several strides to rebalance the horse between the shoulder in and the turn around the haunches, but as they get more competent then you want to ride seamlessly between the movements.
After riding a whole square of the complete exercise, ride large on two tracks and pick up trot. You should feel that the hindquarters are pushing more energetically off the ground. The horse will also seem to “sit” more in the trot, and have a slightly shorter stride with more cadence, slower tempo but still the same rhythm, and overall have that impression of having more power contained within their frame.
I find this exercise doesn’t need repeating too much because it is quite strenuous, but is very useful to do if they feel a bit stiff, or lacking focus in their work. Each horse I’ve used it with has almost immediately felt more balanced in their trot, with more “ping” and bounce to the stride afterwards. It’s definitely a useful tool for my toolbox!