when do spark knocks happen?

Answer 1

When the fuel explodes under pressure due to too low octane rating. The fuel is normally compressed and then will burn when the spark plug ignites the mixture.
When the compression is to high then the fuel will explode before the plug fires, this will be before TDC or top dead center and this will put a force on the piston to return downward before it's ready and you get a knock.

Answer 2

This happens when your car's timing is off. To correct this problem you need to turn the distributor one way or the other to retard or advance the timing.

This will fix your problem!

Hope this helps