They usually have a knob on them that you dial your desired advance to. Say you want 18 BTDC initial so you dial 18 and point it at the mark and if it is at 18 it will show the pointer at ZERO or TDC. If not not you adjust the dist until it is. Same with total. Set the dial to say 36 BTDC, rev it up to whatever rpm max timing is, say 3000 RPM and if the pointer is on zero you're golden and if it's off adjust accordingly.

It takes all the guess work out trying to figure out which mark is which.

Just so you know if your initial is where you want it but the total isn't, changing one changes the other.

Confused yet?

Kevin