Spare response pads. Easy to apply. Pack of 12 pieces of pads.
Responsive systems make your return to the hand. Likewise, response systems are necessary for the yoyo to even start spinning. Response system does not affect whether yoyo is responsive or not (this is given by the bearing and gap size). Every time you throw, play and return a yoyo, response system works with a string.
After a certain period of time it wears out and needs to be replaced. It may be after several weeks or months of yoyoing. You can tell your response system needs to be changed if the yoyo does not get as much rotation with your throw as before. Yoyo also feels like slipping or sliding out of your hand instead of rolling down. Also getting yoyo back to your hand is harder, binds are not tight as before, you need to do several binds to wind up yoyo properly. Almost all of the yoyos today are using thin pads.