You may be confusing this with frame rate adjustment.
Under "Settings > System > Advanced system Settings > Advanced display settings" some Roku devices have an "Auto-adjust display refresh rate" item.
If this is enabled, the Roku circuitry sends the signal output at the refresh rate (frame rate) used by the original source material, commonly 24 fps (frames per second), 25 fps, 30 fps, 50 fps, or 60 fps.
Every time the source frame rate changes, the TV must adjust its display to accommodate the new frame rate, which on many sets results in a short blackout. This can happen when starting/stopping new programs, and when going in/out of commercial breaks that have a different native frame rate than the programs they are inserted into.
If "Auto-adjust display refresh rate" is set OFF, the Roku converts everything to a frame rate of 60 fps internally and the TV doesn't have to adjust on the fly.
The tradeoff here is that movement of things on the screen for non-60 fps sources may not be as smooth with no frame rate adjustment. You have to decide whether this change is worth it to you.
no, I'm not confused.
In the Roku 3810 and 3820, go to settings - display type.
It's all right there and nothing to do with frame rates.
Automatic 1080p
or
Manual settings of 4k dolby HDR TV, 4k HDR TV, 4k TV, 1080p TV, 720p TV
I have changed from "automatic" to "manual 1080p TV".
I can snap a pick if you like...
I'm well aware of the options under Display type.
It was 'I thought "auto" would stop the screen from flashing if/as changing resolutions on the fly within a show/movie' that I meant you might be confused about. That sounds more to me like the transition effect when the tv shifts frame rates. How that manifests depends on the characteristics of the particular set.