The Apple Watch’s battery isn’t as bad as people made it out to be, about on a par with other popular smartwatches. But that hasn’t stopped third-party companies from devising ways to extend the device’s longevity, namely the Reserve Strap.

Even before the Apple Watch was available, the company claimed it could double the device’s lifespan with nothing more than a clever strap. Now that Apple’s watch is out, Reserve Strap has released a video explaining—and proving—exactly how this works.

The $249 gadget—which is only $100 cheaper than the cheapest Apple Watch—charges the device through the wearable’s hidden 6-pin port, which isn’t necessarily meant to be accessible by developers and consumers. Instead, it’s there for diagnostic purposes.

It’s worth noting that the Reserve Strap isn’t currently certified under the Apple Watch accessory program, which means you should buy it at your own risk. If you do plan on picking one up, know that it does indeed work, and actually charges the Apple Watch slightly faster than a regular inductive charger.

Apple hasn’t said how it plans to take advantage of the 6-pin port other than for diagnostic purposes, though if there are opportunities like the Reserve Strap, perhaps we’ll see some accessories down the road that are capable of charging the device. An earlier report claimed the port would expand the capabilities of the Apple Watch, and Reserve Strap is proving that a little moxie can do just that.

There’s no telling how much of an effect the Reserve Strap will have when used long term. But if it provides better battery life, there will no doubt be plenty of interest in similar bands in the future.