Instructors sometimes say to "swing your leg" during waltz, foxtrot classes.

The only way I can swing my leg is to put the weight on the other leg and relax the swinging leg. And while swinging the leg, the foot traces out an arc in the air.

I thought in these dances I go forward by gliding the heel of the forward foot so the body weight is distributed on both feet. But if I glide forward, then I can't swing the leg.

And what is the difference between dancing with swinging legs and dancing with non swinging legs?

Could someone help explain?