I happen to know a decent amount about engines and how they work so here you go. Timing advance is required to make big power because once your spark plug ignites the fuel charge, it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches top dead center (TDC) will allow the mixture to get fully burnt soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur just after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder with the greatest force. If the ignition spark occurs at a position that is too advanced (ex: you have added to much timing) relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing detonation (or knock) and lost power. If the spark occurs too retarded (late) relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, high emissions, and unburned fuel...but is the safer of the two options by far.

 

all that sounds good to me. Some people looking for safetly and not all power will stop sooner than 6* of pull.