The dial is useful on older cars with centrifugal advance - you use it to dial in total timing. It is also very useful if your balancer only has a "0" mark.
In short it let's you adjust when the light comes on instead of coming on as the #1 plug fires.
This applies to old school stuff... say you want 40 degrees timing at 3,000rpm. You'd set the dial to 40 deg and rev to 3k then adjust your distributor position or advance springs so that the timing mark lines up to "0".
No school like the old school.
Wes
In short it let's you adjust when the light comes on instead of coming on as the #1 plug fires.
This applies to old school stuff... say you want 40 degrees timing at 3,000rpm. You'd set the dial to 40 deg and rev to 3k then adjust your distributor position or advance springs so that the timing mark lines up to "0".
No school like the old school.
Wes