Shuffling is a technique of using the glitch that allows to switch between paired rails very fast.
This glitch appears in THPS3, since the new game engine was used. It allows to increase combo multiplier very fast, since every jump adds x1 to multiplier. In Airport player can gain more than x30 while grinding down the escalator. THPS4 didn't change the situation very much.
However, in THUG developers decided to "fix" the bug. They did not remove it, but added a shuffle bonus instead, which adds 100 points for each jump. This way shuffling is not really useful.
How to performEdit
Shuffling can only be performed with paired rails that you can find at top of walls in Suburbia, escalators in Airport, rails in Shipyard, handrails in Skater Island, etc. While grinding, player should ollie towards paired rail and press grind button very fast. Skater will continue grinding the attached rail.