The term Mechanical Trigger refers to the type of trigger design on over and under shotguns that makes it possible to pull the trigger and release a hammer with each pull of the trigger, regardless of whether the first shot fires. Most over and unders have inertia trigger systems. The difference between mechanical and inertia systems is detailed here.

• Inertia Trigger — The trigger system is designed to "reset" the second hammer only when the mechanism is reset by the recoil of the first shot. The traditional reason for an inertia trigger system is to prevent a shooter from inadvertently pulling the trigger and firing the second barrel due to the recoil of the first shot. The time it takes to reset the trigger is not discernable – but is just long enough to prevent this inadvertent pull of the trigger, called "doubling." Over the years, for various performance reasons, many trigger systems on over and unders have been built with "inertia" or "inertia-operated" triggers.
• Mechanical Trigger —  Unlike an inertia trigger, a "mechanical trigger" does not rely on recoil to reset for the next shot. Instead – by design – you get immediate second shot capability with a second pull of the trigger. To accomplish this without creating the possibility of doubling, and at the same time achieving performance-level trigger pull weight and feel, takes an extremely innovative trigger design. 

Two Browning over and under shotguns currently have mechanical triggers. They are the Citori 725 and the Cynergy. These two high-performance Browning shotguns have been fitted with these exclusive mechanical trigger designs to meet the needs of performance shotgunners who desire second shot capability regardless of whether the first pull of the trigger fires the shell or not. Reliable mechanical triggers are esteemed in the firearms industry. Mechanical trigger systems are often desired by expert level clay target shooters who want an elevated level of speed and control. Some systems over the years sacrificed trigger pull and feel to achieve this. Browning trigger system designs overcome this limitation.

The Cynergy features the innovative Reverse Striker ignition system; a striker-based, mechanical trigger that offers the benefits of a crisp feel, reduced locktime and less overtravel than other over and under shotguns. Because inertia is not needed for the hammers to set, the design eliminates the possibility of the second barrel not firing.

The exclusive new Fire Lite Mechanical Trigger is a big part of the Citori 725's performance package. The innovative design features reduced take up, a crisp break and shorter overtravel. Inertia is not needed to reset hammer for the second barrel.

 This diagram shows the mechanical trigger system found on the Browning 725 over and under.

The excerpt below about the trigger function is taken from the Cynergy owner's manual. Many owner's manuals are available for download and are found under the Support tab along the top menu of most pages on browning.com

"Unlike other over and under shotguns, the Cynergy features the Reverse Striker Trigger System. The Reverse Striker is a mechanical trigger system rather than a traditional recoil-actuated trigger system. With the Reverse Striker, after one barrel has been fired the trigger shifts itself into position to fire the second barrel when the trigger is pulled for the second shot. It will not fire both barrels simultaneously. Only one barrel will be fired with each pull of the trigger."