This is a very simple and very effective version of the plank. It’s done using one of your arms set directly out to the side like the outrigger on a canoe. This allows you to target the deep muscles of the core that are responsible for rotational stabilization (the obliques and transversus).
Just get in the normal plank position then set one arm straight out to the side.
In the easier version, the arm supported on the forearm should be under the body to better take up your bodyweight.
Then as you build strength, you can make it more challenging by shifting over so your arm is outside your body “shadow”, creating more tension in the obliques.
Do one side the repeat on the other.
For a different challenge, you can also do this one on one leg at at a time.
This tends to involve the lower abdominal area a bit more.
Overall, this is a very straightforward version of the plank that allows you to target different aspects of your core musculature with a few simple changes in limb positioning.