Another fun one here! Begin shoulder in, after 3 strides, send your horse forward on the diagonal line that corresponds to the angle of the shoulder in. After 3 strides going forward, begin the shoulder in again, on a new line that runs parallel to the long side. Repeat as many times as you can, given the length of your arena. Make sure to do it in both directions. This exercise will help horses that tend to fall through the outside aids in shoulder in. It also reminds horses to rock back after going forward.