Sometimes the instructor has to use his hands to push you into the right position. Another large part is both of you stand in front of a floor to ceiling mirror. He'll be in front of you off to one side, so you can see both him in real life and your reflection at the same time. He moves, and you match, looking at both him and your reflection.
Lastly, you'll never learn a partner dance online, because it's all about the feel of working with a partner.