Flashback Doorway

The Flashback Doorway is quite literally a gate to the past. It requires a person to enter a date and step through the doorway physically, but only the mind is sent to the other time. Also, the user has to focus their thoughts on an individual who existed at that time, so that that person's body can serve as the host of the time traveller.

Fiction

Earthforce

Megatron was in need of extra troops so he built/stole the Flashback Doorway with the plan to collect the Seacons from just before Starscream killed them. The Autobots intended to prevent this, primarily out of fear of what would happen to the timeline if the Seacons weren't destroyed as they were supposed to. Fighting their way past Megatron's guards, Prowl got through the door shortly after Megatron and took possession of his own, younger self's body. With it, he confronted Megatron, who had taken possession of the Seacons' leader, Snaptrap. Prowl KO'ed him, causing Megatron's mind to return to the present, and then watched as Starscream destroyed the Seacons. Knowing there was only one thing more he needed to take care of, Prowl confronted Starscream in order to have his younger self killed as well, as he had been. The destruction of his host's body brought Prowl back to the present as well, where the Flashback Doorway had been destroyed to prevent any other such attempt to alter time. Flashback!