Stand tall with your core braced and a flat back. Place your hands on either side of your head. Take a step forward with your left leg and bend at the knee, slowly lowering your body until your front knee is bent at least 90 degrees, whilst your rear knee should be hovering just off the floor. This is your start position. In one explosive movement, push through your feet jump into the air. Keep your torso upright the entire time and maintain your hand's position on either side of your head. Simultaneously, switch your leg stance in midair. As you land, immediately lower down into a lunge position on the other leg. Continue to alternate legs until all the required reps (or time) are completed.