When you compress already hot air it gets hotter so the issue is with lower octane fuels this heat can ignite the fuel before the piston reaches the desired point of ignition, if this occurs very early in the compression stroke you can easily blow parts into nice pieces as you have th entire rotating assembly going one way and one piston and rod being forced the other basically. The idea with running a lower CR with forced induction applications is that you can force more and hotter air into the cylinder before it will reach detonation as it is being compressed less in the cylinder.
Hey Legz, compression is exactly what it sounds like, air being compressed, and when it is compressed it heats up.
