This plank variation increases the work through the obliques and transverse abdominis muscles.
Set yourself up in the typical plank position with your forearms on the floor and a straight line running down from the top of your head along your spine and to your heels. From here you should bring one hip towards the ground, before using your obliques to bring your hip back up and into the starting position. Repeat onto the other side.
If you imagine a paint brush were attached to one of your hips, you should be aiming to create a semicircle every time you go all the way from one side to the other. Whilst you are moving your hips and rotating through your core you should aim to keep your shoulders as still as possible.