How are you able to create movement in a swing as a closed system?

How can you rock yourself to movement on a swing as a closed system without external forcing? What are you exploiting by being able to move your legs to create swinging movement?|||You are essentially using potential energy. By swaying your feet and body, your muscles put in the work to raise the PE of the system. It is then converted into KE as swing motion. You then add more PE by swinging your feet again. There is external force which is brought in as stored energy (calories) in your body.