Let's start with the basics, just in case.
In an ideal world, your display refresh rate is exactly equal to your media framerate. For example, 25 frames per second (fps) would match a display refresh rate of 25 Hertz (Hz).
In a less-than-ideal world, your display refresh rate is a multiple of your media framerate. For instance, if your media's framerate is 25 fps, and your display's refresh rate is 50 Hz, the player or renderer can simply display each frame twice. This approach works, but it's not always ideal, especially if you want to use motion interpolation on your display.
In a less desirable scenario, your display refresh rate and your media framerate don't match at all. For example, a framerate of 23.976 fps doesn't translate well to a 60 Hz display, which means that some frames will have to be displayed more times than others. This discrepancy can result in stuttering, juddering, or choppy motion. However, you can mitigate these issues using techniques like frame blending.
When you start a video, JRVR attempts to match the framerate and refresh rate, which may cause a brief blanking effect. This blanking is just your display switching to the new frequency, and it's a minor inconvenience, in my opinion.
If you don't want this behavior and prefer to have choppy motion (I honestly can't understand why anyone would want that, but to each their own), you can simply untick the option "Enable Automatic Display Settings changing" under "Display settings" (Tools > Options > Video > Display Settings).