A woodruff key is a semicircular shaped, removable key that fits into a matching keyway cut into a shaft (often the crankshaft), leaving a protruding tab. The tab mates with a matching slot on a device mounted flush upon the shaft (such as the flywheel or clutch), thus preventing the device from freely rotating about the shaft.
Tomos A3 "woodruff key"
The Tomos A3 uses a pin to align the flywheel with the crankshaft instead of the typical half-circle woodruff key utilized by most other moped manufacturers. This design seems to be somewhat more fragile than a normal woodruff key, and in many instances the pin may be sheared or the hole in the crankshaft may be damaged. To help prevent this kind of damage, use the proper flywheel puller when removing the flywheel, and torque the flywheel nut (which is M10x1, FYI) to the specs listed in the manual.
This is how the pin aligns with the notch in the flywheel. Note that usually most of the pin will be in the hole in the crankshaft, and only the ~1mm tall part that sticks
out of the crankshaft will fit into the notch.