A cycloidal reducer is a kind of gear reducer. The essential operation is an input shaft connects to and drives an eccentric bearing. This bearing further drives a cycloidal disk (sometimes known as a cam) that connects to an result shaft.
cycloidal reducer
Usual cycloidal speed reducers, such as these Dojen series Gear rack reducers from Onvio, provide zero backlash by combining a compound 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 certainly transformed into rotation of an 
output shaft at a reduced speed and a rise in torque.
The main benefit of cycloidal reducers is their house of zero or near-zero backlash, which results in high precision and high accuracy. That is especially in demand where highly exact and accurate positioning is necessary, for instance in robotic applications, machine equipment, and comparable applications. Cycloidal reducers also feature rolling instead of sliding contact, therefore experiencing less overall use.