The core muscles participate every significant movement, stabilize your body so they have a very important role. If you fail to train them properly then in a better case you just won’t develop but in a worse case you can also suffer very serious injuries.
Go down in plank on the ground so that your elbows are below your shoulders. Support yourself on the front of your toes with straight torso and closed legs. Keep focused that you contract your buttock muscles, pull in your belly and the pelvis shall be slightly overthrown; you hold it…hold it and hold the position firmly. Then rotate to both sides with straight arms and torso.