The closest physical world analogy is moving in a familiar room in the darkness -- you kind of know where the corners are, and where to find the light switch, so you can move around, and tell, if asked, what's where... But there's no seeing involved.

So, when asked to imagine something, for me the process is akin to drawing a blueprint, and then mentally modeling how that contraption could work in real life, without actually building it. Imagination is certainly involved, but it may not be the kind of imagining the requester assumed.