This is my version of the "dead bug" - it's one of my all time favourite lower back exercises. I find it particularly useful as a starting point with patients who get back pain when standing, walking or running. If you find it more challenging than it looks, you're in good company!
For those who have mastered the basics, there are lots of variations that can make the exercise harder. Do make sure that you've gone over the original progression first, even if you're already strong, though. Good technique is everything with this exercise.