It is good practice to keep your yo-yo in top working order. Not doing so may result in damage to the yo-yo, yourself, your property, and/or others. For fixed-axle yo-yos it is fairly important to change the string regularly. You do this when your string starts to look dirty and worn out where the slip knot is attached to your hand, and/or where the yo-yo is attached to the axle.
In ball-bearing yo-yos you still have to keep track of when it is time to change your string by looking for the same signs of wear as with a fixed-axle yo-yo. You also may periodically have to lube the ball-bearing to keep it in top working order. Allowing the ball-bearing to dry out can damage the bearing. By allowing the bearing to spin without a lubricant in it, the metal pieces of the ball bearing wear each other down, causing it to spin less effectively and slowing down your yo-yo. Occasionally debris from the wear down of the bearing can lock it up, keeping it from spinning.
Most ball bearing yo-yos have a replaceable response system. A response system is the method the designer of the yo-yo used to make the yo-yo return to your hand when you tug on the string. Typically, these response systems consist of a rubber coated o-shaped sticker that sits directly around the bearing on each hub of the yo-yo. Some manufactures use a cork cutout with an adhesive on the back for the same purpose. Regardless of your yo-yo’s type of response system, it is important to replace it when the yo-yo starts to lose its responsiveness. If you don’t, you will lose your ability to control the yo-yo and make it return, wind effectively, and spin to its maximum potential.