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Bring your other arm into a "High-V". Keep your core muscles engaged and your base leg straight. Stay balanced using your entire body. Remember to smile.
The hand not holding your leg should be closed into a fist. Pulling the other arm into the “High-V” is the correct way to do the move stylistically, but it also helps you keep your balance. Your knee should be unlocked but straight, and your body should be tight.  Locking your knee can be dangerous. Keep it very slightly bent to avoid injury. Keeping your core tight will help you maintain your balance while holding the heel stretch. Focus on the muscles in your lower body and the posture of your upper body in order to stay balanced.  The foot on the floor should be firmly planted. Your body weight should be evenly distributed through the heel, ball, inner, and outer sides of the foot.  The thigh and calf muscles in your standing leg should be held firm and straight. Tighten these muscles to keep from wobbling or jutting your hip out. Your back should be straight with your chest and chin held up. Keeping the upper body strong and lifted will help you stay balanced. When done properly, the heel stretch should look easy and graceful.