Springer heavy pellets?

[Shed tuner]

Thanks guys, I'm still none the wiser though, I could not really see what was happening in the video as it's just a spring moving? Any chance of a brief explanation in layman's terms?

watch the back of the piston.. you see it accelerate, stop, bounce backwards, and the go further forwards than where it originally bounced from, to it's end of stroke resting place.