The spark timing determines how much compression is in the cylinder at the moment the fuel/air mixture is ignited. So, while it's technically true that it's the spark timing that's being adjusted, the thing that is being controlled for is the compression.
The spark timing determines how much compression is in the cylinder at the moment the fuel/air mixture is ignited. So, while it's technically true that it's the spark timing that's being adjusted, the thing that is being controlled for is the compression.