After a person has refined himself and is free of sin, he is able to receive prophecy on condition that he prepares himself to do so. Prophecy does not involve the soul actually leaving the body, for that would merely be a dream like any other, but, instead, takes place when the soul is still within the body. During prophecy the prophet disconnects himself from his physical body and connects to his spiritual self.

Prophecy comes to a person after he has refined himself and is free of sins:

This explains the phenomenon of prophecy. For when a person is within a spiritually refined body, free of the filth of the evil inclination, of the drives of his foundational soul, and with no sin that pollutes any of the roots of his soul, he can, if he focuses on it, connect to his upper spiritual source and cling to it.

The soul detaching from the body: Total focus on the spiritual:

However, even though one is fit to [prophesy], he must strip his soul entirely and separate his soul from physicality. Then his soul will be able to reconnect with its spiritual source. But this separation, spoken about in the mystical works dealing with prophecy and the spirit of holiness, is not actual separation, in which the soul leaves the body, as it does in sleep. If that were the case, prophecy would be no different from a normal dream. In truth, when the spirit of holiness rests upon a person, his soul is within his body and he is awake; it does not leave the body. The separation is that the prophet totally detaches himself from his thoughts and his imagination, which stem from his foundational animal soul. He stops [his animal soul] from imagining, thinking about, or pondering any matter related to this world, as if his soul left him. Then the power of imagination within him transforms, rendering an image as if he has ascended to the higher levels of existence, climbing stage by stage to the roots of his soul until he vividly imagines his upper source. All of the lights that he sees with his imagination are embedded in his consciousness in a way parallel to the way he imagines this world even though he does not apprehend them through sight, as is known by the science of nature. (Rabbi Ĥayyim Vital, Sha’arei Kedusha 5)

Further reading: For dreams and their meaning, see A Concise Guide to the Sages, p. 415.