A cycloidal reducer is a type of gear reducer. The basic operation is an input shaft connects to and drives an eccentric bearing. This bearing further drives a cycloidal disk (sometimes referred to as a cam) that connects to an output shaft.
cycloidal reducer
Typical cycloidal speed reducers, such as these Dojen series reducers from Onvio, provide zero backlash by combining a substance reduction cam with preloaded, full compliment needle bearing cam 
followers.
The cycloidal disk (or cam) has lobes or teeth that engage with some form of cam follower, typically a pin or needle bearings. The eccentric rotation of the cam is usually changed into rotation of an output shaft at a lower life expectancy speed and an increase in torque.
The main benefit of cycloidal reducers is their property of zero or near-zero backlash, which translates into high precision and high accuracy. This is especially in demand where highly precise and accurate positioning is necessary, for instance in robotic applications, machine equipment, and Electric Motors comparable applications. Cycloidal reducers also feature rolling instead of sliding contact, hence experiencing less overall wear.