Negative tachyons are particles that can increase a speedster's speed by siphoning positive tachyons from the Speed Force.
The Time Bureau had at least one encounter with negative tachyons, learning that "instead of speeding you up, they slow you down." It also became known that if negative tachyons were to get anywhere near a timeship's "time drives" the reaction would vaporize the ship.
When Nora West-Allen time-traveled to 2018 she used a device she found in the Flash Museum archive to harness negative tachyons, preventing her from entering the Speed Force, in order to appear unable to travel back to her time, hoping this would allow her to stay in 2018 and spend time with her father.
At another point, Team Flash realized Nora had time-traveled using the Negative Speed Force when Cisco Ramon discovered radiation from negative tachyons, which he realized didn't just offset positive tachyons but rather siphoned them from the Speed Force like a parasite.
After his death in the Anti-Monitor Crisis, Eobard Thawne became a spectral entity made up of negative tachyons. He had the ability to possess humans, but in doing so would lose access to the Negative Speed Force unless he could force the host to fully submit to his will. He possessed Nash Wells, so Cisco tried to use the positive tachyon device (PTD) to bombard Nash's body with positive tachyons to force out Thawne, but Thawne's tachyons were multiplying faster than the PTD could get rid of them. However, Barry and Cisco were able to force Thawne from Nash by going into Nash's mind using the MAD 2.0 and making Nash face his emotions.