This is a good total body exercise. You should be able to do a plank with a good neutral spine (not sagging) to do this exercise.
Keep your plank and bring one knee as far as you can while keeping it off of the mat. Then return to the starting position (perfect plank). Alternate sides.
To lower the intensity let your knee or foot rest for a second before returning to perfect plank.
I challenge you to give it a try!
Live Good Fitness.com