I was recently asked about improving this, and thought I should share this exercise.
The ten metre trot circle in the first corner, done well, helps to create and store power, and ensure you are setting off as uphill and ‘set up’ as possible.
Then straighten, rebalance and engage, and think really upwards/forwards across the diagonal. A good image is to think of a jumbo jet taking off! It’s about lift not speed, you want the horse to have time to take the bigger steps, not to accelerate and run.
For horses just learning, a few steps of difference is fine. Once they’ve got the hang of it you can ask for more, with a rebalance in the middle if necessary.
I went to a Mary King demo years ago and she told us that she does that rolling ‘R-R-R-R-R’ noise (like a person imitating a big cat growling!) to teach her horses to open and extend.
I can’t do that, however hard I try, so I settle for a sotto voce “Hssss” noise to give them the idea. They soon learn to associate the noise with the action, in both trot and canter.
Sometimes doing bigger rises (as long as you keep your balance) can also help to show a young horse that you want a bigger trot.
The most important part of this exercise, apart from the good start from the ten metre circle, is the rebalance at the end, well before the corner. I find that knowing that you have another ten metre circle to do helps this, and prevents riders from winging around the corner (which flusters horses and can lead to them being unwilling to open up across the diagonal, because they don’t trust that they’ll be correctly rebalanced before the corner.)
This whole exercise is also a test of how good your half-halts are, and whether your horse is in front of the leg.
Remember that the change of bend should come as you meet the track on the new rein, NOT at X (this was stressed by Isobel Wessels at the Hartpury IEF.)
Obviously you go across the diagonal as straight as possible, but you change to your new inside leg – outside rein aids etc as you meet the track, not before.
Of course you can also do this down the long side, to ensure straightness. If the horse sucks towards the boards/fence, do it a metre or two off the track.