Here, Parrish wants you to stabilise the shoulder joint while taking the top reaching arm and scooping it underneath your body between the floor and your rib cage — without sinking into your supporting shoulder.
In your forearm plank, step your feet hip-width apart.
Internally rotate your left arm so that your fingertips are facing the elbow on the right.
Turn your heels to the left as your toes turn to the right. Your weight should be on the inside of the right foot and the outside of the left foot.
Float your right arm up towards the ceiling to find yourself in a side plank.
Inhale and take your right arm and scoop it underneath you between your rib cage and the floor. As you turn your right shoulder down towards the floor, lift out of your left shoulder and armpit.
Exhale and return to your side plank.
Repeat 10 times before returning to your regular forearm plank to repeat on the other side.