When launching the game, should come up with a confirm controller section, as it would on docked.
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https://gbatemp.net/threads/reversenx-flag-your-console-as-handheld-or-docked.543625/
https://gbatemp.net/threads/overclock-sx-os.547569/
MAKE SURE TO THOROUGHLY READ EACH THREAD AS INSTALL INSTRUCTIONS AND SPECIFIC SUPPORT IS PROVIDED ON BOTH
Above is links to what you need and discussions on how they work. Install the files on the root of your SD card. To use, you need to go through homebrew menu. To do this, activate a game and hold R while it's launching. Select sys-clk, a menu should appear, click enable from first menu. Will then have to manually adjust overclock by selecting title id section and looking for Witcher 3. Select Witcher 3 and adjust clocks accordingly.
Reverse NX is similar. Launch from homebrew menu and will come up with it's own menu. You have two options. System-wide flags or individual title flags. To access both, it will give instructions on the menu. Recommended is individual title mode. Enable docked-flag, press button for exit, then launch Witcher 3. Will have to repeat this process every time, unless you choose system wide option.
Very bad description, but if you need further help, I'll try my best.
Thanks Mate. You legend