Board Thread:The Flash discussions/@comment-28354683-20160501172546/@comment-34302681-20180111204909

My theory is that Eobard Thawne’s negative speed force always matches Barry Allen’s speedforce. So whatever time Eobard Thawne is in that Barry is in he will automatically match the speed of Barry. This explains why he got stuck and lost his connection to the speed force after killing Barry’s mother. To explain why he can still run at super speed when traveling to a time where barry never was, it’d make sense that whatever version of Barry he last interacted with is the Barry that his speed matches.