In Episode 19 of Season 5 of The Flash, series introduce negative speed force and how can Speedsters access it. The Negative Speed Force was created by Eobard Thawne as a negative version of the Speed Force. It is an extra-dimensional field of negative energy that eats away at the regular Speed Force like cancer and can consume any speedster connected to it by touch. Like Barry Allen with the Speed Force, Thawne himself is also the generator of the Negative Speed Force with every step that he runs. He used it to infect and corrupt Barry, in the hopes that the Negative Speed Force would consume him.
According to the show, Eobard Thawne showed Nora Allen how to access Negative Speedforce. She wanted a way to go back in time to meet her parents and to defeat Cicada. During the first half of the episode, she is not able to access it, this is due to her emotions coming in her way. But, later on, she uses her emotions and pain that her father is still mad at her, she gains access to the negative speed force. At the End of the episode, we see that Nora’s lightning has turned Red instead of Purple and Yellow. So now Nora Allen has Become Reverse XS.
In the Post-Flashpoint universe, and after the events of DC Rebirth, it is revealed that the Negative Speed Force exists and that Eobard Thawne is still the creator and generator of it. Thawne can tap into it as normal speedsters can with the normal Speed Force, but with its own unique energy signature. The Negative Speed Force grants the users all of the abilities that the positive Speed Force does, such as the power to move at vast superhuman speeds, as well as superhuman physical characteristics, regenerative healing, enhanced mental capabilities and perception, lightning generation, molecular control, and the power to traverse throughout both time and dimensions.
Some abilities the Negative Speed Force grants that the positive Speed Force does not are: the ability to negate Speed Force users’ powers, the power to absorb memories from others, and the power to alter one’s age at will, as well as tearing out Negative Speed Force from others without killing them. Also, Negative Speed Force users have the ability to accurately time travel without devices. The color of the lightning varies from red, black, and purple. Unlike positive Speed Force lightning, Negative Speed Force lightning does not act like normal electricity. It is far more destructive and can cause objects to explode on contact.