In social dancing, we often don’t care how the legs look and we go based only on feeling. This is one way to do it and if you bend or straighten at the right times, it will feel good and also probably look good.
But for “performance dancing” or controlling your aesthetic, knowing when to straighten and bend your legs not only feels great and gives you the extra strength/stability but also helps you draw beautiful lines.
There are only 3 rules (to bending and straightening your legs):