Bird-Dog is an excellent exercise to stabilize the low back during upper and lower extremity movement. The main target of the Bird Dog is the erector spinae. It’s not that difficult to do after a little practice to get the balance right. It is helpful to use a mirror to help you with form adjustments.
It looks easy, but it´s not. Let´s see the usual mistakes or problems with the next pic.
I have to say this was not on purpose :) Usually, my first set of 30 reps (15 each side) is for stabilizing. Flipped disks are that funny :)
You should not lift your leg above hip height. This will help to avoid upward rotation at the hip. Do not allow the shoulder to tilt upward. If you cross the “red line”, you should adjust instead keep counting wrong reps.