Normally, shuffling reduces your speed by an amount proportional to your initial speed (see JSRF Speeds Chart). A negative shuffle utilizes a particular setup to cause a shuffle to increase your speed instead, slowly building up to extreme levels.
The only known way to do a negative shuffle is during a mission against a CPU player, e.g. City Rush: you must skate sufficiently ahead of your opponent, turn around and skate backwards into them. If you shuffle a little before the point of impact, you will be knocked down and will emerge into a negative shuffle. In that state, your speed will continue rising until you exit the shuffle.