If you use a speed increase on the exact moment a flag "Stack" is applied past 6 the speed reduction does not apply for the duration of that stack. It is the reason that rogues randomly get full movement speed past 6 stacks at lvl 100 through hitting burst of speed on the stack timer.
This is never deliberate. Never, nobody in RBGs or Twink games has ever abused this on purpose but say it doesn't happen would be a fairytale. It's very easy to this accidentally and the effect of it can be game changing. It is best to alert players to the general area in question so people know it exists and they try (Where possible) to actively not do it.
Just to reiterate. This is almost impossible to abuse, it just happens pretty randomly if you aren't careful.