Kewlstart is a coined term for an extension of Loop start

Loop start

In telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...

 (FXO and FXS) signaling which adds Disconnect supervision

Disconnect supervision

Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....

 through the use of an Open Switching Interval (OSI). In addition to the operation afforded by loop start, the CO (central office) signals the terminal (user) end that the distant party has hung up. The CO switch will remove battery voltage from the loop for about 250 ms, within 6 seconds after the far-end party disconnects..

Most real phone switches support this feature. It is generally required for getting hangup notification.
The other mechanism of POTS

Plain old telephone service

Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

 disconnect supervision

Disconnect supervision

Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....

 is battery reverse. In order to indicate disconnection of the far-end party, there is a polarity reversal of tip and ring conductors of a telephone port.