Nintendo Switch Sharing

As mentioned in the comments, a short press of the Sync button instantly turns off a Joycon or Pro controller. There are no other shortcuts for this.

Games which behave nicely will update the number of controllers that they support, automatically turning unused ones off for you anyway.

Edit: A Family Group is not required for this; you only need to add your account to the other switch.

From Nintendo's website:

Non-Primary Console:

• A Nintendo Account can be linked to Nintendo Switch consoles that are not designated as the primary console, however, some functionality differs from when a Nintendo Account is used on a primary console.
• While using a non-primary console, you can still use the Nintendo Account to access the Nintendo eShop, purchase and download software, and under certain conditions, play your downloadable software.
• While using a non-primary console, you must have an active Internet connection to play downloadable content.
• If you lose your Internet connection while playing downloadable content on a non-primary console, your game will pause after a certain amount of time; however, once you connect online again, you will be able to resume from the point you left off.
• While using a non-primary console, downloadable content can only be started by the user that purchased the content.
• When using downloadable software on a non-primary console, your game will pause if your Nintendo Account is used to access downloadable software on any other Nintendo Switch console.

Yes, you can share games with another switch owner by adding your account to his switch. He will only be able to play the games using your account and cannot play the same game that you are currently playing at the same time. However, there are a couple of workarounds for this (I have not tested these, but have heard other people discussing them):

• If you want to both play the same single player game by yourselves, you can take your switch offline and his switch shouldn't pause since it doesn't detect that you're playing the same game.
• If you want to play a multiplayer game together online, you must start the game as a different user. Because you are the membership owner, any other user on your console can use the membership, even if they're not a member of the family group. On his console, only your account will have access to online. This method can also be used for single player games, but you won't have access to your user's save file if you are playing as a different user.