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(Pocket-lint) - If you think you know everything about Apple Watch, think again.

Despite all the tear-downs and breakdowns and coverage, a hidden hardware feature on Apple's smartwatch has just been uncovered by an accessory maker that plans to capitalise on its discovery. If you slide away Apple Watch's bottom band, you'll notice a 6-pin port within a connection slot covered by a metal hatch.

Apple hasn't acknowledged this "diagnostic" port on its website or at any press event. It doesn't even include a tool for getting access to it, meaning the secretive port is likely just for Apple's own internal use, but that hasn't stopped Reserve Strap from developing a battery strap that'll take advantage of the feature.

Here's what Reserve Strap is saying: "We've developed and tested a completely rethought design that takes advantage of the 6-pin port underneath the band slide of the Apple Watch. This port hadn't been deciphered by anyone," explained Reserve Strap, while noting it wants to shift development focus to the new method.

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Reserve Strap's battery strap, which fits both the 38mm and 42mm case sizes, is expected to start shipping in the autumn. By utilising the Apple Watch's 6-pin port instead of wirelessly charging, the $249 strap can allegedly achieve a higher charge capacity and quicker, more efficient charging times.

It's unclear if Apple wants accessory makers to use the port for charging docks and accessories, so keep in mind it is possible the company might disable the port with a future software update, leaving Reserve Strap with no choice but to fall back on old plans that involved inductive charging.

Writing by Elyse Betters. Originally published on 4 May 2015.