Rather nifty little piece of kit there!

I think the little lever on the RHS pulls the pin thru the block enough in a straight line to allow the block to then rotate away.
When is in battery the pin (I believe) goes deep enough into that recess/lug in the LHS side of the action & will hold the block closed, making it unable rotate due to the pins length binding itself in the recess until the lever pulls the pin out of the recess again - well that's my theory Darwin!
Great find Lancaster.