Bill,
I have checked with the actual owner of the tool and it seems that:
Undoing the knob allows the blade to move out, pulled by an internal tension spring. The blade can then be pushed back in against the spring tension until the protruding spigot reaches the end of the slot at which point the blade is retracted. So no use as a plane as the blade retracts as soon as it digs in. So maybe for protecting the surface.