The main function of the eccentric bottom bracket is the tightening of chain or belt as a transferable element. When it rotates, the tightening to a required level occurs. The eccentric bottom bracket is used for frames where a rear multi-speed rear hub is required or at tandem frames to tighten the connecting chain. Tightening with the help of eccentric bottom bracket is always placed on the opposite side from the crank set which means that on a classic frame the chain is tightened from the left side and on tandem frames on the right side. (See the picture). This is a very elegant solution of tightening when it is not necessary to screw a special offset bolt which is very often a source of other service activities to keep it working correctly.
Photos and video