The third-party Reserve Strap is gunning for the not-so-secret port on your Apple Watch.
While originally intending to use inductive charging, third-party Apple Watch band Reserve Strap has been redesigned to utilize the hidden port inside one of the watch's strap grooves. That port can apparently be used for charging.
From the Reserve Strap website:
Our engineers have been able to independently confirm that the 6 pin diagnostic port underneath the Apple Watch case can be used for charging. The Reserve Strap will take advantage of this using a simple, retractable connector instead of the previous inductive charging cradle design.
The port on the Apple Watch is currently covered, but the Reserve Strap is supposed to come with a tool to allow access.
While a number of third parties have announced bands for the Apple Watch, the Reserve Strap is currently one of the few to offer extended capability to the watch, and for good reason. It's entirely possible that bands like the Reserve Strap just won't work. Even if they can get the port to work now, future software updates may disable their ability to use it. In fact, it's highly likely that updates will do that, as Apple won't like third parties making unauthorized use of the port.
In any case, the Reserve Strap is up for pre-order for $249. It is set to begin shipping out in fall 2015.
Source: Reserve Strap