That was a good explaination of mechanical advance but you left out the vacuum advance. As your RPM's increase you need to advance (or lead as you say, I like that skeet description) the spark ahead of the piston reaching top dead center. While the piston is travelling faster as the rpms rise, the speed at which the fuel ignites remains the same, so you have to in effect pre-ignite the chamber so that the piston barely crests the top of its stroke to take full advantage of the burn. This is accomplished with mechanical advance. Your engine pulls air into itself as a result of this burn process through other cylenders. The carb or throttle body restricts how much air can be drawn in and at idle creates a vacuum in the intake manifold. When you open the throttle up and allow air into the engine the vacuum drops until the engine can catch up and pull more air than the carb or throttle body will allow, whcih creates a vacuum agian. This just so happens to be a good reference to how much load the engine is under since a low vacuum would mean the engine cant draw air in as fast as its allowed to do so. By attatching an advance to the vacuum signal, you can lead the spark even further under no load which results in better fuel economy and retard the timing under load to get that spark closer to happening right near top dead center and take advantage of a full stroke of power.
