THESE ARE MAINLY USED BY MOUNTAIN BIKERS. THE REQUIREMENT TO HELP KEEP THE CHAIN SLACK FROM BOUNCING IS DUE TO THE TERRAIN IN WHICH THE BIKE IS RIDDEN ON.
Many chain devices are situated around the crank area and fit closely to the chain and Chainring, only millimetres away. Should you get some bounce to the chain, the chain device prevents it from leaving the drive of the chain ring. Should you lose the chain the pedals can be unpredictable due to the loss of drive.
E13 and MRP are probably the best known for this style of chain device. They come in various mounting systems so they are versatile and as the user of a chain device would normally only have on chain ring the device can be fitted with a bash guard on the outer side of the crank. This is to help keep the chain encased and to protect the chain and chain ring from rocks and roots that may otherwise damage them.
Chain tensioners and guides are also used on mountain bikes. These fit on the chain stay part of the frame between the crank and the rear axle. The chain threads through the device on the underside of the frame and is lifted slightly from its natural line. This adds a little extra tension therefore removing bounce from the chain.