1) Start off by laying on your back with your legs fully extended.
2) Raise your torso (at a 45° angle) and legs (whilst extended with a slight bend in your knees) off the ground. Keep your arms bent at the elbow and to the side of your body. Keep your back straight and your core tight.
3) While keeping your torso as stationary as possible, bring your left knee up towards your chest.
4) Simultaneously, bring your right arm forward leading with your elbow. Keep a slight bend in your elbow throughout. You're supposed to mimic a "sprinting motion" with your arms.
5) Bring your knee and arm back to the starting position and alternate between your legs and arms - right knee, left arm; left knee right arm.
Start out slowly as you learn the tempo of the exercise until you get the hang of it.
Your goal is for the entire movement to be seamless and fluid as you transition from one knee and arm to the other.