Hint A18 - Rod locks
This clever little part "locks" any classic K'NEX rod into a connector, to reduce the possibility of the joint becoming broken.
It is generally used where rod/connector joints are under higher-than-usual stress eg in the Son of Serpent coaster and the K-Force series of sets.
To use the rod lock:
1. Join a rod to a connector, as usual.
2. Slide the rod lock onto the rod, with the orientation shown in the photo.
3. Slide the rod lock so that it fits snugly over the place at which the rod fits into the connector.