To do the Plange Plank, start in a plank position from your forearms and toes with your feet together and your elbows under your shoulders. Make sure your body is in a nice straight line from your head to your heels.
Then slowly round your upper back as you tuck your hips under. Draw your belly button in toward your spine as if trying to “vomit up a hairball.” Try to tuck your hips under toward your ribs as you round up. Do not simply stick your butt up in the air. Also, make sure to keep your elbows under your shoulders.
Hold here trying to engage your core as hard as possible. If you do this correctly, your abs should almost instantly start to shake. You should be engaging your core as hard as you can.