When dancing the waltz or foxtrot, what goes into making the dancing soft and light? Thanks

Impartng the quality of "softness" requires leg strength, the capacity to use the knees and the feet to absorb energy.

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In ballet, the concept is referred to as "ballon."

Ballon ; For example, during the grand jeté, the dancer may appear to hover. Physically, his/her center of mass describes a parabola, as does any projectile. Observors have limited ability to reckon center of mass when a projectile changes its configuration in flight. To do this the dancer extends their arms and legs, which camouflages the fall and leads the audience to perceive the dancer is floating.[1][2][3]. This is interesting and is another way of thinking of hovering,extending the length of time we are in the air before lowering.