“Phase-cut” dimmers work by taking the line input power (120V “house power”) and modulating the signal to reduce the power to the load. By “chopping” the signal, the load experiences a lower voltage, resulting in a lower light output. The two most common “phase-cut” controls are forward phase controls and reverse-phase controls, each with their own benefits and limitations. Leviton residential “phase-cut” dimmers offer the ideal solution for controlling the light level for any activity within the home.
Please see the link below for detailed information on the differences between forward phase and reverse phase controls.
https://www.leviton.com/en/docs/Dimmer-Forward-and-Reverse-Application-Note.pdf