Many people are familiar with Transfer Switches for backup Generators. A transfer switch is essentially a switch that switches between 2 loads. The purpose of a transfer switch is to provide power in the event of a power outage or if a utility source fails. There are Manual Transfer Switches where you would physically have to throw a switch in order to turn on the backup Generator. Then there are Automatic Transfer Switches, which will automatically switch over to the backup Generator.
Within Automatic Transfer Switches there are Open Transition Switches and Closed Transition Switches. The most common is Open Transition, also known as break-before-make Transfer Switch because it breaks contact with one load before making contact with the other load. That would cause a momentary interruption of power before the load switches to the backup power source. Closed Transition Switches are known as make-before-break Transfer Switches, meaning the backup power source switches on before breaking contact with the existing load. This is most commonly used for critical loads such as Elevators or Hospital equipment.
This is a photo of a Closed Transition Transfer Switch used to feed critical circuits in an Institutional Facility. The momentary interruption of power is less than 1/6 of a second .