Despite rear derailleurs with tensioning devices, complex chain deflections and sophisticated rear triangles: As soon as a chain moves on a cassette for shifting operations, the chain needs a certain length to bridge the different numbers of teeth. This shifting capacity of the rear derailleur and chain can lead to wild rodeoing of the chain, usually on the chainstay. 

The chainstay can be wrapped with an old tube, for example, or special damping foils can be used. The makers of Rali have come up with a new approach: the chainstay protector consists of damping plastic lips and there is a magnet in the front area. Without chain tension, the magnet pulls the chain towards it and prevents chain slap. This clip shows whether it works: