Tear the stems from the kale and chop up the leaves. Place in a colander and rinse thoroughly under cold running water, drain and set aside. In a small bowl combine the soy sauce, vinegar, garlic, sugar and crushed red pepper flakes to make the garlic sauce. Heat a large wok or pot over high heat, add sesame oil and the kale, tossing to stir fry for 5 minutes until the leaves are slightly wilted. Add the garlic sauce, stir to combine.