How are rotary encoders used on a keyboard?

A rotary encoder is a device that you can twist to perform actions. Many rotary encoders can also act as a button, though they usually require more force to press than a switch would.

A rotary encoder twists clockwise and counter clockwise. It lends itself to actions that you'd otherwise need to repeatedly press a button for or to actions that have both a forward and a reverse direction. For example:

• Volume control, to turn the volume up or down;
• Scrolling, to scroll up or down a page or document with Page Up and Page Down;
• Tabbing, to go to the previous or next browser tab with for example Control Tab and Control Shift Tab;
• Window movement, to go to the previous or next window with Alt Tab and Alt Shift Tab;
• History scrubbing, to perform undo and redo;
• Scrolling horizontally by word, with Control Arrow Left and Control Arrow Right (if you hold down shift while turning the encoder, you can select words too!);
• Scrolling through search results, with Find Next and Find Previous when you map the encoder actions to your text editor's shortcuts.

Of course, you can program them to do anything. I've seen people program common text snippets so you can enter text with the simple turn of the encoder.

Most encoders can also function as a button, you can simply use the keymap to assign an action to the button press. You can for example have an encoder mute on press, and control volume with rotation, but it can also be another action entirely.

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An encoder knob placed on a Kyria rev 2.