This is the aqua version of the Plank Knee to Chest exercise. This type of exercises I prefer to be done in the water, where your body is safely floating and your legs kicking against water resistance, than on dry-land.
Planks are great for core muscles but they also work other muscle groups. Although performing it in the water is safer, you'd still want to keep a good posture during the exercise.
Lay on your front and hold the noodle down, under your chest with straight arms. Keep your legs straight, slightly below parallel.
Bend twice with inhale and twice with exhale
Use your abs to bend your legs and your glutes to kick back.
Tip: Try to touch the noodle with your knees
Plank Crunch in the pool