Black frame insertion
Black frame insertion is a method of displaying games running at 60 FPS on 120Hz monitors.
It simulates CRT flicker, which is necessary for the human eye to perceive fluid motion. Without it, the sample-and-hold method used by LCDs manifests as motion blur to our eyes. For LCDs, running at 120Hz with black frame insertion every other frame gives you 60Hz CRT motion quality on 60fps content. This is especially effective on strobe-backlight gaming monitors (e.g. Nvidia LightBoost, EIZO Turbo240, BenQ XL2720Z Blur Reduction) that often only enable motion blur reduction backlight strobing only at 120 Hz.
CRT monitors can use a 120Hz refresh rate to sync to 30kHz (240p) resolutions. However, 240p at 120Hz can create motion blur, due to having twice as many frames being drawn on screen and overlapping. The solution is to draw a black frame every other frame. At 120Hz that essentially brings it back down to proper 60Hz. The issues are that brightness is halved and any frame drops or synchronization issues are very noticeable, so you may need to adjust your display's controls to compensate.
In RetroArch, there is an option for black frame insertion in the video options in the menu. This makes it draw an extra black frame for every frame and it succeeds in making the motion smooth at 120Hz. There is an option to set the vsync swap interval if you want to double frames instead. The "refresh rate" setting should be set to 60Hz (monitor refresh rate/2) for accurate dynamic rate control in these cases.
120Hz with software inserted black frames has less latency than doing so on the display's hardware. It also has less latency vs. displaying straight 60Hz due to the decreased frame times between vsync in emulators. It also doubles the performance requirements of an emulator to maintain vsync for the same reason.