The word toggle comes from a pin passed through the eye of a rope to hold it in place—or, as it is more commonly used today, a switch that alternates between two settings, like on and off. A toggle can be a physical switch or a software one, but it is more often used in computer technology as a way to enable and disable features—the way that you would click your caps lock button to activate or deactivate your keyboard shortcuts. The toggle is a common control in forms, within full pages of information, and in more modal experiences.
When designing toggles, consider how the UI will look to the user when it is fully configured (On) and when it is partially configured (Off). Toggles should be clear and concise, utilizing visual signifiers such as movement or color to indicate state. They should be easy to read, especially for users with low vision, and they should make it abundantly clear what each switch will do.
It is important to avoid using a toggle where the user will need to save or confirm a new configuration, such as in long form fields, since it could delay their ability to get immediate results. Instead, it is better to use a checkbox or another type of button.
Savvy teams view the Feature Toggle inventory in their codebase as inventory that comes with a carrying cost, and strive to keep it as low as possible. To achieve this, they frequently perform test runs with all toggles flipped On and then, when they decide to roll out a toggle configuration change, they will also test the fallback configuration with all toggles flipped Off.