Here is the skinny of it all taken from the Walther catalog:
By integrating a Single Action Anti Stress trigger into its design, the Double Action P99AS helps prevent unintentional “reflex” firing in stressful situations. After the slide has been pulled completely to the rear, the trigger remains in the forward position for the first shot. This prevents inadvertent firing not only in the double action trigger mode, but also in the single action mode. As a result, the trigger travel is .55” long in the anti stress mode with 4.4 lbs of trigger force that helps to prevent inadvertent firing. On all subsequent shots, the trigger travel is reduced to .314” with the same trigger force for classic P99 quick reset firing.
Here is a visual explination of the trigger system:
The P99 Quick Action pistol with uniform short trigger pull and independent safeties is the choice for both law enforcement tactical teams and competitive shooters. The P99QA hammerless pistol is capable of being disassembled without the need to pull the trigger. Simply remove the magazine, clear the chamber, press the takedown button and slide the takedown catch to remove the slide. The trigger mechanism incorporates a striker system that is partially pre-cocked. When the trigger is pulled, the striker becomes fully cocked and released, firing the pistol. With approximately .31 inches of trigger travel and 8 lbs. of trigger force, the short trigger pull and reset provides quick shot delivery, especially in competitive situations.