Slide back with your knees slightly until your forearms reach the ground. Slide forward and push yourself forward to put weight on your hands again and push up. Repeat. This exercise is very tough. Do only as many as you can do with a clean form. As a variation, you can do the backward movement and go on your knees while moving forward. Instead of a slider, you can use a blanket, a towel or any other piece of cloth.
Push Up with Knee to Elbow
Come into a high plank position. Lower down while you move one knee to the same elbow. Push yourself up again and bring the foot next to the other one. Repeat and change sides.