Look at the front of the can, and the bolt, in both animations.
At the front of the HE can, you can see see a ledge that the stock can doesn't have. There is also an additional o-rinbg on the bolt.
When the bolt moves forwards, the additional o-ring on the HE bolt seals against the ledge in the HE can. This seals off the air input in to the can. This is the difference that makes the HE bolt kit fire from a fixed volume of air. That isn't necessarily to say that the HE bolt kit is necessarily more efficient than a well tuned stock bolt, but like Apollo said, it minimises the effect of poor tuning.
For instance, if your dwell was set longer than it needed to be to get your ball up to speed, you could be losing efficiency with either bolt kit. However, the HE bolt kit fires from a fixed volume of air, so any loss is minimised. Once the air in the can has been released, it doesn't matter how long your dwell is - how long the bolt is forwards - once the can is empty, no more air will be released.
With the stock bolt kit, air will constantly free flow in to the can and out through the bolt for as long as the bolt is forwards, so excessively long dwell settings will just waste air.