The short over simplified version is that in a motor you want the fuel mixture to burn not explode. To hot a temperature, to much compression, to much timing will cause the mixture to explode. Race gas has a hirer octane, that means it burns slightly slower and is more resistant to 'exploding' , this allows you to use more compression, more advance etc.... these are the things that give you more power, the race gas just allows you to make more power without grenading the motor. Since it 'burns' slower it makes slightly less punch than lower octane pump gas which is the reason in can make less power when used in a motor that doesn't need it.