Switchhook flash

Hook flash or flash (known in the UK as "recall") is a button on a telephone that simulates quickly hanging up then picking up again (a quick off-hook/on-hook/off-hook cycle). This action can signal the telephone exchange to do something. A common use of hook flash is to switch to another incoming call with the call waiting service. Another use is to indicate a request for voice conferencing, for example, a user may use a procedure like the following to initiate three-way calling:

The second "flash" signals the Central Office Switch to link the two active conversations, so that all three parties are connected to the same logical telephone line.

In contrast to PBX conferencing systems, the two calls are joined at the Central Office switch, rather than at the customer premises PBX.

The switchhook is the device that senses whether the handset or receiver is in its cradle. The term "flash" originated from the cord circuit of the early telephone switchboard that telephone company operators used to connect calls. The calling party and called party each had an indicator light on the cord circuit. When the subscriber cycled the telephone on-hook/off-hook, the light would flash. Actors in old movies often demonstrate the method, seeking the operator's attention. The user does a tap-tap-tap. When an operator comes on the line, the actor says, "Hello? Operator? We've been cut off." Then the operator attempts to reestablish the connection. The flashing light of this equipment of bygone days is the origin of the phrase "flashing the switchhook".

Centrex telephones added a hook-flash button in the 1960s after some users incorrectly attempted the attendant-recall function.

...
Wikipedia