Here is an extremely advanced side plank variation that I haven't seen before. It is a feet elevated side plank with single leg abduction. The huge difference is that your feet aren't resting on something like a bench or box so you have much less support from your lower body, so your core and upper body have to work significantly harder to keep your body in proper alignment. I won't lie, this is tough! My obliques were on fire!
Set yourself up so you are in a side plank position with one hand on the floor and both feet up the wall. Generally, you want to position your hand so it is below your shoulder. I have my hand so it is slightly ahead of my shoulder, and this position feels great for me.
Once you are in position, tuck your ribs towards your hips (close the space in your midsection), contract all of your core muscles (essentially all of your muscles but your limbs), and slowly lower your leg towards the floor, bring it back to the wall, reset and repeat. Aim to deeply inhale (360 degrees of air around your spine) and exhale the entire time.
The whole point of this exercise is to keep your body in proper alignment, and for your body to resist extension, rotation, and lateral flexion. WOW do you ever feel your obliques! I like to use Tony Gentilcore's ''canister'' analogy. Your ribs and pelvis should remain stacked like a canister vs falling out of alignment and looking like scissors. This will help shut off the sympathetic nervous system, which is ideal.
Start out by just holding the side plank without abducting your leg. If this goes well, incorporate the leg abduction movement.