How do I wake up Siri on my watch?

I own an Apple Watch. I used to own an iPhone 6 Plus and to wake Siri up on my watch all I had to do was make sure that the phone was unplugged and say 'hey Siri' directly into the watch.

I recently upgraded my phone to an iPhone 7 Plus and it has a feature that allows Siri to 'listen' even when it's not plugged in.

This is now causing a problem when I try to wake Siri only on my Apple Watch. Now that my iPhone 7 Plus is 'listening' for the 'hey Siri' command, it is taking precedence and starting Siri on the phone. I've tried shouting 'hey Siri' really loudly at my watch but no matter what I try, it it is only being activated on my phone. I've even told Siri on the phone to do things like 'start a timer on my watch' but it only seems to take commands for the phone.

How do I wake Siri on my watch now that I have an iPhone 7 Plus?