|“||It's the, uh, messed up British Men of Letters thing they used to get inside people's heads.||”|
The mind-link device is a device created by the British Men of Letters to link the minds of two individuals, primarily for brainwashing.
Following the capture of Mary Winchester, British Men of Letters operative Lady Toni Bevell uses a mind-link device at the British Men of Letters compound to enter Mary's mind and over time brainwash her into the perfect assassin for the organization. Eventually, Toni is successful in fully brainwashing Mary to the British Men of Letters side.
After Toni reveals that she can't undo the brainwashing, Dean requests that she get him into his mother's mind like Toni did. Toni tells him that she'd need her rig which is at the British compound. However, Toni suggests that she can "cobble something together" using materials from the Men of Letters bunker. After arriving at the bunker, Toni gets what she needs from the storeroom and is able to put together a version of her device which she uses to link Dean into Mary's mind with Toni monitoring them from the outside. Just after Dean manages to reach Mary, the link is violently broken when Arthur Ketch kills Toni and removes the device from Dean's head.
In order to save Dean from the Apocalypse World Michael's possession, Sam pulls out the mind-link device with the intention of using it to link into Dean's mind and get him to kick Michael out. With Castiel hitching a ride with Sam's mind, the two are able to link with Dean, ending up in a void surrounded by Dean's memories. After locating the right memory, the two enter it and find Dean where they are able to convince him of the truth and help Dean lock Michael away inside of his mind.
|“||These electrodes synch your delta waves with Mary's, forming a psychic link. But to enter her psyche will take a certain measure of concentration and skill and as there's no time to teach you how to reach the necessary therapeutic dream state I'm banking on this. Hypnotic agents laced with a potent sedative.||”|
The cobbled together version of the device appears as a pair of old pieces of machinery with switches and gauges and electrodes running from them. Completing the machine is a computer tablet that monitors the brainwaves of the two users. The electrodes are attached to the heads of both participants and synchs their delta waves together, forming a psychic link. However, in order for one person to enter the other's psyche, the linker generally has to reach a therapeutic dream state. Barring the ability to reach such a state a cocktail of hypnotic agents laced with a potent sedative injected into both participants can put them into the needed state. However, when Sam used the device, he was able to link into Dean's mind just by activating it. By touching Sam's shoulder as he used the device to link with Dean, Castiel was able to "hitch a ride" with Sam into Dean's head through the device.
Once linked, the amount of manipulation of the linked person's mind appears to be related to the skill of the person linking to them. Lady Toni Bevell was able to use the device over several sessions to get into Mary Winchester's head and brainwash her into being an assassin for the British Men of Letters, locking the real Mary's mind behind psychic wards that even she couldn't break with Mary effectively hiding behind the wards. When Dean used the device, he was able to link in behind the wards, but could only witness what Mary was doing in her own mind and communicate with her, but could not actually manipulate her mind like Toni did. After Dean reached Mary and got her to stop hiding, it broke the brainwashing while Dean was violently disconnected when Arthur Ketch ripped the electrodes from his forehead. When Sam and Castiel used the device, they linked into a formless void surrounded by Dean's memories which Castiel was able to make audible using his angelic abilities. Once they found the correct memory, they were able to enter it and interact with Dean, though they were unaffected by his looped existence, instead witnessing it happen around them. In this state they were only mental projections of themselves and were able to interact on an even footing with Dean and Michael.