Agreed. It's just another way of illustrating the way the force works...

Vader tortured Han on Bespin to lure Luke there. Before that though, Luke senses that his friends will be in danger even though the pain hadn't taken place yet. Vader understood the force and took advantage of this bizarre paradox.

Han says "they didn't even ask me any questions". This is because Vader knew the pain alone would be enough for Luke to sense it (at a previous moment in time).