Very simple and very effective – works your entire core, including your abs and obliques and the QL. I like to use something to take the stress off my wrists when I do planks. In the video below I’m using the Perfect Pushup handles to allow my wrists to stay in a neutral position and rotate a bit. I type and mouse for a living so that’s really important to me.
Just get down into a plank position with your feet wide and hands close, then put a hand up in the air above you, rotating into place off the balls of your feet, using your hips and shoulders to drive the motion. Gently return to the plank, switch hands and repeat on the other side.
For an ab exercise like this I recommend you do 10 on each side to begin and work your way up to 25 on each side. If that gets too easy you can elevate your feet, or suspend them in straps or bands to make it more difficult. Go slow, be careful, and protect your back and joints.