It's not a feature really, but a characteristic of the spring based on material and gauge. Several mods recently, like the DrawTube, and apparently this #20 are using a steel spring. Not ideal, as a steel spring has lower electrical conductivity, and potential slower response to overcurrent. Typically a brass spring, or plated brass spring is the best option. The gauge, number of coils, etc. determine how much current the spring can handle, so it's important to have the proper balance of safety vs. resistance.
If you need more explanation, the atmizoo site does a good job of it.
Roller/Dingo: Battery Spring | atmizoo vaping modware | shop
edit: Here's a video of how a "hot spring" is made. The spring is wound compressed, then stretched to dimension. When the spring reaches an overcurrent condition, it collapses.
Hot Wound Coiled Spring – Duer Carolina Coil – Hot Wound