
The trick with using Hey Siri on Apple Watch is that the screen must be lit up on the device. Aside from that, the feature is always enabled for use (unless you turn it off). That is how it’s different from enabling Hey Siri on iPhone, which is always waiting for a voice command from anywhere within voice shot as long as the feature is turned on to begin with (at least with newer model iPhones).
This means you have two ways of activating “Hey Siri” voice commands on Apple Watch; by raising the Watch wrist, or by having the screen on and then issuing a command.
Raise the Apple Watch Wrist and Say “Hey Siri”
This is the most common method for users to activate Hey Siri on Apple Watch. Simply raise the wrist you’re wearing the Apple Watch on, and say “Hey Siri”, followed by your voice command.

Tap the Watch Screen, Then Say “Hey Siri”
Tapping on the Apple Watch with a finger, nose, or other appendage, so that the screen is lit up, then allows you to follow with the traditional “Hey Siri” command prefix to activate the feature.

Source Url and Image: How to Use “Hey Siri” on Apple Watch