As I remember, there are spring loaded balls inside that grip the line. To make it work, you pushed down on the plunger under the bail, thus relaxing the balls. The line was then fed up thru the balls and out the hole in the plunger. Releasing the plunger then allowed the balls to grip the line and hold it tight. Pulling on the line just made the balls grip harder.
Kind of like the old Chinese finger grip thing.
Mike