It’s possible to use the Apple Watch completely hands-free, useful in an emergency or when wearing gloves.
The Easter egg involves the fact that anytime the Watch face is displayed, it’s listening.
Simply raise the wrist, wait briefly for the watch to wake up, and say “Hey Siri”. When you see the familiar waveform then speak away. Siri will understand almost anything as usual and do your bidding.
This trick puts hands free operation in the palm of your… er, on your wrist.