The way mugwort (Artemisia vulgaris) helps with vivid and sometimes lucid dreams, and altared states of consciousness is that it contains small amounts of the substance Thujone, which is a mild hallucinogen. It is the same chemical present in wormwood, which gives absinthe its mind altering properties.
In very small amounts, such as a cup of mugwort tea, it will not send a person on any sort of trip or journey. In fact, most people would not notice any effects while awake from so little. However, it may aid in meditation-based exercises such as astral projection, and can cause more active dreams -- inducing lucidity for some, and possibly triggering a more emersive experience for some (such as more realistic sensory perception within the dream).
Beware seeking sleep paralysis, though, as it can be a terrifying experience. Yes: some may use it as a way to asltral project, but when the brain has not woken up the body, and the mind is aware of one's surroundings during a hypnogogic state, some really bad experiences can be had -- especially if there's a little bit of dream boosting thujone making the waking dreams more active.