A knock sensor is a device used in internal combustion engines to detect and prevent engine knock, which can cause damage to the engine if left uncontrolled. Knock, also known as detonation, is an abnormal combustion process that occurs when the air-fuel mixture in the combustion chamber ignites spontaneously and uncontrollably, producing a knocking or pinging sound.
The knock sensor is typically located on the engine block or cylinder head and is connected to the engine control unit (ECU). The sensor works by detecting the high-frequency vibrations produced by the knock and sending a signal to the ECU. The ECU then adjusts the engine timing and fuel delivery to prevent knock from occurring.
There are two primary types of knock sensors - resonant and piezoelectric. Resonant knock sensors work by using a tuned resonant circuit that vibrates at the same frequency as the knock. Piezoelectric knock sensors work by using a piezoelectric crystal that generates an electrical charge when subjected to pressure or vibration.
The benefits of knock sensors are numerous. By detecting and preventing engine knock, knock sensors can help improve engine performance, efficiency, and longevity. If left uncontrolled, engine knock can cause damage to the engine, including burned pistons, damaged bearings, and warped cylinder heads.
this exact sensor with a mounting stud can be found here Knock sensor kit
Overall, knock sensors are a critical component in modern internal combustion engines that help prevent engine knock and improve performance, efficiency, and emissions. By detecting and controlling knock, knock sensors can help ensure the longevity of the engine and reduce the environmental impact of internal combustion engines.
Here's a snippet of how we use these in a real world application monitoring each cylinder. 
We run reasonably low ignition numbers to start with, this is to determine the knock threshold and even out the cylinder frequencies first. Once we've evaluated the data we can build out a knock threshold table and start to introduce timing to the motor. If and when there's a knock event that peaks past a set knock threshold(knock floor) it will immediately pull out timing per the cylinder that register a knock count. We can then detune that cylinder at the exact RPM and pressure. Very brief run down but should help those wanting to understand more about the safety features most aftermarket ecu's offer. We highly suggest Link engine management. https://ae-race.com/