What are Eccentrically Cycloidal Gears?

Designing and building a 3d Printed 15:1 eccentric cycloid gearbox / actuator to increase the torque from a stepper motor. CAD and STLs are available at: These are the supplies i used: NEMA 17 stepper: Arduino Mega: 5mm steel rod: Thin wall bearings: and: PLA filament: