A cycloidal reducer is a type of gear reducer. The Gear Reducers essential operation is that an insight 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 result shaft.
cycloidal reducer
Regular cycloidal speed reducers, such as for example these Dojen series reducers from Onvio, provide zero backlash by combining a substance reduction cam with preloaded, complete 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 transformed into rotation of an output shaft at a reduced 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 popular where highly exact and accurate positioning is required, for instance in robotic applications, machine equipment, and similar applications. Cycloidal reducers also feature rolling as opposed to sliding contact, therefore experiencing less overall wear.