Evil Road Spirits! They love to ride, but they can't stand bells. Road spirits can't live in the presence of a bell, the constant ringing drives them insane, they lose their grip and they fall to the road. (Have you ever wondered how potholes are formed?) Anyway the bell has served its purpose.
If you've picked up a bell on your own, the magic will work. But if your bell was given to you, the power has been doubled, and you know that somewhere you have a special friend helping to look after you. When placing a Guardian bell on your bike, the closer to the ground the better. Usually the lower part of the frame is good. Don't give those little critters a chance to get on board.