My Sempert & Krieghoff drilling has separate rifle barrel cocking by way of a lever on the left side. To uncock it, I simply hold the lever all the way down and pull the trigger to release the sear, allowing the lever to take over the spring tension from the sear, then slowly return the lever to the original uncocked position.

This is in essence the same way I would uncock a hammer gun, by holding the hammer back while releasing the sear and allowing the hammer to slowly move forward to the uncocked position.

I have never owned a drilling with the cocking feature you describe, but I own three with the lever system and they all work the same way.

I would put a fired case or a snap cap in the rifle chamber before experimenting, to guard against damage to the hammer or to the firing pin, should you accidentally release it.