Retractable threading tool - videos

[metric_taper]

The tool in metric_taper's video is an eccentric with a spring return.

From reading the comments, the eccentric is 0.060 offset so it throws 0.120 disengage in 180 degrees of travel.
What confuses me, is once it's triggered, the spring will push the eccentric, which seems to not be what I would expect. It appears to be a snap action. Just seems an eccentric would not work that way. I assume it's slightly overcenter during the thread run, and tool pressure on the material keeps that in place. OK, maybe that's why is snaps away, the force is from the tool pressure, and the spring just keeps it moving. Just seems it would get stuck once the tool pressure is gone.
I'll have to build one to see how it works.