To answer your question:
A stock trigger unit will usually rely on a direct mechanical hold between the trigger and first sear.
The common practice for L96/APS-2 it to have a lever hold the piston at 90deg.
That lever is pivoting on an axis that is in the middle of the arm. The other end it held in place by a long lever that moves up-down. Moving it up puts it in the path of the first sear. Moving it down let the first sear pivot and release the piston.
In the stock unit, the safety is preventing that lever from moving down when engaged. The trigger pushes on the other tip of the lever to push it up, which makes the lever pivot on the pin and pushes the other end down (and firing).
In the upgraded unit, the safety is not present because the point where the trigger apply force on the second lever is moved in the place the safety was. This makes the force required to move the second lever a lot less (and making the trigger pull lighter). The size and ratio of the levers is also changed a little to make the force from the spring easier to hold. The trigger box is also made from two metal or fiberglass plates, making it a lot sturdier in a front-back axis (reason why stock boxes break).
VSR-10 stock triggers work a bit different because the first sear pivot point is attached to the second sear, moving the whole first sear down when the trigger is pulled. This makes a lighter stock pull, but the sear has to be angled back because of that. The joint between the piston and sear is at 45deg and uses the piston a lot faster because the sear has to slide on the piston at every shot.
The upgraded VSR-10 trigger works in the exact same way as L96/APS-2 triggers. That is the reason why it comes with a special piston to replace the stock one, the joint is made at 90deg and thus very sturdy.
Originally Posted by Drake
Damnit, don't make me add "no discussing temporal paradoxes" to the rules or I'll go back in time and ban you last week.