Thumping the rifle on the ground is indeed one "method" of getting the second barrel to fire on a single inertia trigger gun but I would certainly not call it a "trick"!

I have a O/U rifle which has a single inertia trigger and in the event of a misfire with #1, if you really do know the "trick", you can fire #2 without thumping anything!

I guess I should add that in a lot of shooting with that gun I have never had a misfire in either barrel!