Great for your prep for the beach!
With this exercise, you are combining the side plank with leg adduction hitting both the core and inner thigh at the same time. It is harder than it looks. But it is a great exercise. Build up to it if you can't do it right now by doing both exercises separately and then start combining them and add reps.
Assume the side plank on the elbow or hand and slowly lower your bottom leg down keeping your toes pointed up and to the front. Keeping your core tight and hips up and straight, contract your inner thigh muscle (adductors) and bring your foot up to your top foot. Remember to remain tight and straight and don't forget to breathe.
Give it a try and let me know what you think.
Master Sergeant, U.S. Army Retired
NASM Certified Personal Trainer & Weight Loss Specialist
Graduate Student In Exercise Science At Cal U.
Dempseys Resolution Fitness