<< My question is, what the heck is "knock"? >>
Engine knock is a sharp metallic noise, sometimes referred to as engine clatter or pinging, caused by the pre-ignition of fuel as it is compressed in the cylinder, milliseconds before the normal spark plug firing. This condition typically occurs during acceleration, such as merging into highway traffic, or under heavy load conditions, such as pulling a boat or travel trailer. Under knock conditions, a vehicle will experience a reduction in power output as well as reduced fuel economy.
The higher the octane, the more resistant the fuel is to pre-ignition... this is why higher compression engines require higher octane fuel because the fuel would ignite because of the pressure which would be a little too early in the cycle, therefore throwing the engine out of sync and reducing power.
More info on knocking:
Several factors can contribute to, or result in, engine knock. These factors include outdoor temperature extremes, altitude, humidity, operating under heavy engine loads, time elapsed between routine tune-ups, vehicle age, and manufacturing and engineering tolerances . The most common cause of engine knock is improper tuning of the engine, where the engine timing has deviated from the manufacturers design specifications. This problem is easily resolved through proper routine maintenance and tune-ups.
As a vehicle?s engine ages, the octane required to prevent knock may increase. This condition is referred to as Octane Requirement Increase, or ORI, and is the result of normal engine wear. In instances where you experience occasional or moderate knocking, your first line of defense is to try a higher octane gasoline to accommodate this Octane Requirement Increase of your engine.
Severe knocking, which cannot be remedied by using a higher octane grade of gasoline, demands your authorized mechanic?s attention.