Degree marks indicate 2.5 degree of crank advance.

It snaps onto the distributor, so when the clamp is loose it can't slip off

To Install:
Remove old plate and bolt.
Snap new plate around distributor and clamp as standard.
Find 0 degrease at 1000 rpm and mark the distributor shank with a punch. Now readjust to your typical spec at 1000 rpm and count the marks. Now you will be able to see the differenced while testing or reset back to standard without a timing light.