Contrary to some reports, shipping Apple Watch units do feature a hidden diagnostic port inside the groove of the bottom strap connector slot. It looks a lot like Lightning I/O, bit it's not Lightning.
Featuring a six-dot brass contact, it's believed to be aimed at service technicians for diagnostics purposes, while potentially providing direct access to the Watch OS and demo software installation on demo units in Apple Stores.
This tutorial explains how you can access the 6-pin port using just a sewing needle. Before we get to it, keep in mind that doing so could impact the water resistance of your Apple Watch and might void your warranty.