Double action means that there are at least two things going on when you pull the trigger. Meaning when you pull the trigger, it is cocking the hammer (or the striker in the P99) and releasing the sear which allows the hammer (or striker in the P99) to fall and hit the striker, which hits the primer on the cartrige. (On the P99 you have a striker which is spring loaded and when the sear is released, the stiker moves forward to impact the primer on the cartidge)
Single action is when the hammer (or stiker on the P99) is already cocked and so all the trigger does is releases the sear to allow the hammer or striker to impact the primer.
Double action only is when you have double action all the time.
So on the P99 the gun starts in DA mode and when the first round is shot, the slide retracts, ejects the spent cartridge, loads another round and cocks the striker, thus the gun is now in single action mode.