Dr. Harrison Wells agreed to help Hunter Zolomon steal the Flash's speed due to the latter's threat on his daughter's safety. A concrete plan started to form when Team Flash captured Russell Glosson; Wells later took a further step by getting a brain tissue sample of him in order to study him, consequently killing Glosson in the process. Wells recorded his journal entry 1-14 on his next gen watch that, by studying the Turtle's brain matter and the encounter with the Reverse-Flash, he completed a device that could harness the Flash's speed when he tapped into the Speed Force, which he installed behind the Flash's white emblem of the Flash suits. It proved to have worked when the Flash went to face Joseph Monteleone for the first time; Wells successfully transmitted some of the speed and later on gave it to Hunter Zolomon.
Later, this device was modified to transfer the energy of the tachyon device to Barry's emblem for convenience and to conceal it against Hunter in order to improve his speed without having to put the device on; allowing the transmitter to independently function as a miniaturized tachyon unit. After Hunter captured Wally however, the transmitter was used to steal all of The Flash's speed in exchange releasing Wally, turning Barry Allen into a normal human and curing Hunter of his cellular degeneration caused by his use of the Velocity drug.
In order to get Barry out of the Speed Force once he rescued Wally West, Cisco inserted "gold-tooth" technology into it to serve as a "tether" for Cisco to retrieve Barry and Wally. However, when Barry was given a test by the Speed Force to battle the now-enslaved Hunter Zolomon, Barry had to use the transmitter to temporarily destroy Zolomon in order to escape him.
Another version of the transmitter was used on Barry when Team Flash attempted to overcome the negative tachyons Nora had pumped into her system to slow herself enough that she couldn't enter the Speed Force to return to her time, she and Barry recreated what the latter had done with Supergirl in order to return to his Earth. However, the team had to stop them, as Wally had learned that they risked altering the timeline in the wrong way if they sent Nora back to the future right then.
Years after the device was used to steal Barry's speed, Team Flash created an idea to develop a weapon to stop a new villain, "Cicada". In order to build it, Barry and Nora traveled back in time in order to retrieve the necessary pieces for it to work; a vial from the transmitter included. Unfortunately, in a battle with Zoom, the vial was damaged and Barry and Nora had to seek help from Eobard Thawne in 2015 to fix it before traveling to another point in time to build their device.