Apple advertises Siri as one of the headlining new features in the forthcoming macOS Sierra software update. Siri on the Mac performs in much the same way as it always has on an iPhone, iPad and iPod touch, except on the Mac tetherless 'Hey Siri' functionality is currently unsupported.
On iOS, tetherless 'Hey Siri' requires Apple's embedded M9 motion coprocessor to monitor microphone input without taxing the battery. Since the battery isn't a major concern with Macs, the decision not to implement 'Hey Siri' on Sierra is eyebrow-raising. As it turns out, you can easily simulate 'Hey Siri' on Sierra without resorting to dark magic, using a simple trickery.