Side plank meets clamshell for obliques and hips at once.
Keep your hips high the whole set so your obliques stay loaded, not your shoulder.
The side plank keeps your obliques loaded in a continuous hold while the clamshell adds a moving element, the top knee opening against gravity with the feet together. Holding the diagonal line and moving at the same time is a harder brief than either job alone. One side at a time, your core keeps the hips high while the hip does its reps.
It layers hip work on top of the oblique hold. Your core has to keep the hips lifted and still while the top knee opens and closes, so you train the side of your trunk and the hip together instead of separately.
Yes, feet together is what makes it a clamshell. Only the top knee opens, rotating from the hip, while the ankles stay in contact. If the feet separate you have turned it into a different exercise.
Your hips have dropped, shifting the load onto the supporting arm. Drive the hip back up into the straight diagonal line before every knee lift, and end the set when you can no longer hold it there.
