Hey B, it's called a 'rock and roll' hinge.  Basically you employ that kind of motion to get it down flat and to bring it back up again for sitting.  When in the seat position It usually involves bringing the base up (reducing the angle between the base and back) to release a catch which then allows the back to roll down flat.  You just reverse this till it clicks to lock the seat upright.As per the pics if you bolt it to the 2 boards which make up the base and back for the seat when folded up - one part to the edge of the bottom board and one to the edge which meets it from the back rest.  It looks like in this case the third board folds down and doubles up over the back rest (I guess it just has some normal door type hinges on it).   R