Place a BOSU on the floor, ball-side down. Don't have a BOSU? You can do this variation without one too.
Start in plank position with your hands at the edges of the BOSU. Bend your right knee, twist your torso to bring the knee a few inches behind your left arm, lifting the leg as high as you can. Return to plank position and do a push-up.
Switch sides and bend your left knee. Twist your torso bringing your knee behind your right arm. Return to plank position for another push-up.