He isn't actually slowing down the trigger pull, he is putting a new stronger spring on the end of the pin ware many people glue over the old one or replace it with part of a pen. By increasing the strength of the spring on the end of the pin that the trigger pushes back on it allows more pressure to build up before releasing the air in the tank resulting a a more consistent release speed no matter the trigger pull speed because the pressure on the spring must exceed the air pressure pushing on the part sealing the tank for the tank to open. I know Dan beaver (I think) did a much better job of explaining that a while back but I cant find the thread
It's still not the spring that initially opens the tank. The spring itself still isn't strong enough to overcome the strength of the air pressure. If it were, putting that there would be the same thing as potting the trigger. What is happening is, is that the spring compresses as the trigger moves back. Once the spring cannot compress anymore, the force your finger exerts on the trigger, goes directly to the firing pin and pulls it back. Once the initial force from the air pressure that was keeping the firing pin in the forward position is relieved, the spring snaps the pin back.
Yeah, that was the original intent of the design. Once the valve actually opens, the spring helps to snap the firing pin back. Some people prefer just fusing/gluing the trigger to the firing pin and pulling really hard, especially in plugged pump blasters that need more force to open the valve anyway. Although the idea of upgrading the spring at the same time is intriguing.
I've found that the stock spring cant even overcome the power of the spring the pushes the pin forward, and actually limits the airflow a bit. A considerably longer and stronger spring fixes this problem. But even the stock design is more consistent than potting the trigger. Although, you are wrong on one point. It's not the spring that actually opens the tank. The trigger actually pushes on the back of the pin through the middle of the spring, and opens the tank. Once it opens the spring snaps the pin back.