That would slow down the application and release of the advance mechanism...that's all it would do. We used to do that on the vacuum line for the transmission modulator valve to keep them from "shuttle" shifting. We would take a glass fuse, bust the glass, and then drill a tiny hole in the metal end of the fuse. Then we'd stick that in the vac line as a restrictor.
Don't have a clue why anyone would want one in a vac. advance line, unless the bike was just too powerful for them at take-off... :smilie_happy:
However...now that I think of it, the reason we did it was because the car usually had burnt/mis-adjusted valves, and the vacuum signal would just be bouncing it's butt off on a gauge. The restrictor would calm the pulses down.