When the cardinal directions have been learned, by placing her muzzle at each point in successive on-demand trials, it is time to move on to other body parts. Next in the series is the "shoulder", representing exactly that - the front shoulder. "Shoulder-left", and "Shoulder-right", instruct the horse to place her shoulder (left or right) onto the target. This generally requires at least one lateral step into the indicated direction.  And upon touching the target, the mission is accomplished.... "X!"

In the beginning...

This is a long way behind where Sierra is, but it is important to understand this one particular point... one's significance must be established. I am the source of your favorite treats, your food when you are hungry, your water when you are thirsty. You will touch your muzzle to my hand in response to the word "hold". There is no other option to obtaining what you want.  I will respond "X".