I don't own S2 but do own a couple of K95s which also use the Jaeger locking system, only difference being it is for single barrel. The pin is used to release the locking block. After removing the barrel you press in the pin which allows the key to open further which releases and allows for the removal of the locking block.
Interesting system this Jaeger block. You can take the barrel (or barrels in S2) out of the receiver and slide the lock into the barrels to provide a solid, bolt action strength lock up. You could literally fire a round of by banging the firing pin with a hammer and the lock up is strong enough for it, no receiver needed. It is quite popular in break action rifles in Europe and is used by Blaser, Merkel, Haenel and probably a number of other makers. Bolt action strength in a break action rifle.