Hi Tom! You're not alone. :)

To achieve this version of the plank, squeeze your shoulder blades together, and then rotate your pelvis back. To make things clearer, picture your pelvis as bowl of water (I encourage you to look it up). A posterior pelvic tilt will spill water out of the back of that bowl. This commonly known as 'tucking the tailbone in'. This is the position we're striving for to increase core activation.