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. With Harry's enhancements from using the tachyon device, Barry was able to successfully outpace Zoom. After Hunter captured Wally however, the transmitter was used once more 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.
It's unknown what happened to the transmitter since, but it was presumably damaged or destroyed in the second particle accelerator explosion; unless it wasn't attached to his suit when they recreated the accident that originally made him a speedster. However, if the device was indeed attached to his suit (which was destroyed) during the explosion, it's possible that when he received a new suit from the Speed Force identical to the one he had before; the Speed Force may have also generated a new transmitter along with the rest of the suit.
After he encountered the Speed Force itself, Barry's speed was enhanced even further, so much so that he could beat Zoom without tachyon enhancement.
In order to get Barry out of the Speed Force once he rescued Wally West, Cisco repurposed the device by inserting "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 take Nora back into her time by recreating what Barry did with Supergirl in order to return to his Earth. However, due to the negative tachyons Nora had pumped into her system beforehand in order to prevent traveling back, this attempt failed.
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.