You should be swinging the kettlebell behind and above your knees, bending more at the hips with a slight bend at the knees and a strong, straight back. (mindbodygreen.com)
Move back to the starting position by bending at the knees while simultaneously leaning your torso forward at the waist, keeping your back straight. (crazygain.com)
There should be right angle between the thigh and the leg and the other leg should not bend at the knees. (yogicwayoflife.com)