First of all, I want to point out that this is not a way to enable some sort of secret handheld mode. This solution is more of a “think outside the box” solution and it only works efficiently for a few minigames. The trick is the following.
This is what portable mode normally looks like:
So what you need to do is the following:
And that’s it. So while you keep holding the Switch with the joycon that is still attached to it, you can use your other hand to play with the free joycon. By the way, this also works for Mario Tennis Aces Swing Mode (or, I guess, any other game that requires using the joycon this way)
Of course, this trick wouldn’t be needed if NDcube had implemented a way to only play the minigames that only use buttons when it detects that the system is being used in handheld mode. But well, we all know that probably won’t happen, even though they did disable the rumble based minigames if you have controller rumble disabled at system level.