This is hard to explain without drawing a diagram but the basic elements are a: a sugar glass and b: a homemade large mousetrap. The glass sits on top of the mouse trap. The trap is painted the same color as the table covering and lies flat on the table so it can't be seen from the audience. The trap release is tied to a black fishing line handled by a member of the stage crew from the wings. At the moment of firing the gun, the crew pulls the string, the trap "fires" and smashes the glass.