They will switch on when the user compresses the switch and will remain on only for as long as there is pressure on the switch.
Once the pressure is removed they will switch off. For example; a door buzzer or an electric drill.
Operators of telephones use the switch to answer the phone and the line will remain open as long as the switch is pressed down.
One of the main reasons for choosing a momentary switch will be for operator safety.
Momentary switches are used on heavy-duty roller doors; the operator must old down the switch when the door is opening or closing, if there are any obstructions to the doorway, the operator can quickly stop the door from closing preventing an accident.