The Boston Waltz was characterized by a blending of a more traditional rotation with a walking step. It was often danced in a close, body to body hold. It often, in practice, devolved into simply stepping only on the downbeat in every three (Step-two-three; Step-five-six). It worked better with fast tempo Waltzes.
This first clip shows an interesting contrast: the ballroom is dancing a traditional Victorian-style rotary Waltz, when the principle couple comes in and demonstrates the Boston.
I will then follow with some other examples. The last one is an example of people just stepping on the downbeats.