This is the third in a progression of exercises, I'll post the link to the full set in the comments.
From any point on the
long side of the arena, the rider will ride a quarter turn on the
haunches and add one to two more additional steps. Before the
horse loses rhythm or balance, ride 1-2 steps straight, then immediately resume 2-4 steps of the turn. You can continue in this manner,
interrupting the turn with 1-2 steps straight to regain activity and
balance. The walk steps should not get too long and on the forehand.
After training the turns, it is often helpful to ride some active trot or
canter work, such as lengthening, or frequent transition within the
gait.
As with any exercise and movement that we teach the horse, in the
initial lesson, we do want to repeat the exercise until the horse โgets
itโ. Once the light bulb has gone on in his mind, then the exercise
can be integrated into the work program on a regular basis, but taking care never to drill the horse in this or any exercise.