This is a system that has been developed by Air Arms over many years. It is a valve system that sits between the rifles main air reservoir and the barrel. When you pull the trigger, the valve opens for enough time to allow a blast of air to move from the reservoir and down the barrel pushing the pellet along. What is clever about the knock open valve system is that when the reservoir is full and has high pressure, it only opens the valve for a fraction of a second, but when the air pressure is lower, it allows more air to pass. This way, as long as you have air between about 105bar and 185bar (on a rifle like a S400), the pellet should always leave the barrel at a consistent speed.