Hip movements in Salsa come from the combination of using both legs.
When you step upon one foot, you will immidiately move all your weight and body to that side(or front, or back). Do keep your standing leg(knee) straight though. And keep the standing side from shoulder, rib cage, hip, knee and a back of ankle in a straight line. Allow your upper body weight settle upon that hip once you get there. Don't purposely move your hip. In the mean time, you will position the weightfree leg in such a way that your knee is bent toward the front where your body is facing. Only the inside of ball of you foot in contact with the floor on the weight free side. You will quickly notice your hip rotate to the back all by itself.
Exercise all the above techniques slowly in front of a large mirror. Gradually pick up the speed to the Salsa musics.