Astral projection is not actively imagining. It is something done in a trance-like state. That is why some people suggest practicing putting one's self into sleep paralysis. Basically, it induces a deliberate lucid dream, in a sense, though people who practice it believe it is an actual experience -- but that comes to belief, and not provable fact.
Here's the thing: Going into a trance can induce a hypnogogic state. That is, your brain is awake but the portions associated with dreaming (such as your hippocampus, which is strongly associated with mapping both memory locations and physical locations) become active in the dream-like sense. It's the same reason why, when someone wakes up during REM sleep and they're still groggy, they have a tendency to see moving shadows on the way to the restroom. Or why people experiencing sleep paralysis see and hear things which are not really in the room: They are awake but the brain is still dreaming.
The feeling is immersive, and there is ability to travel at will. I have never experienced astral projection, but I do have very vivid and lucid dreams on occasion. Some of them can be quite the experience!