The overhang marker is a coloured section unique to each line in each weight. We have cast each weight of line and marked where the optimal point is to release the cast, for each weight, in every range, one by one.
The overhang marker is a brightly coloured section between the head and running line that tells the caster when to release the forward cast. Many people slip line into their false casts, take too many false casts and then release. This means they have too much 'overhang' which is the amount of thin running line between the rod tip and the thicker belly of the line. The overhang marker prevents the caster slipping too much line into their false casts making perfect casts every single time.
2. Don't all lines have this?
No, most lines have a different coloured running line to the head. Or, some have a 'haul zone' which is a harder coating between the head and running line. Neither of these help in telling you when to release the forward cast. On every line, in every weight, the amount of overhang you can get away with is different. We have measured that section on each line in each weight and coloured the exact section. That is very different to a generic coating applied to all lines in all weights to prevent the line wearing out (haul zone.) And a different coloured running line only tells the user where the head starts, the user then has to figure out how much overhang they can get away with which is a very advanced skill, most professional casters have instinctively.