Instead, to invoke Alexa, you had to tap the Tap. But that's no longer the case: Amazon just announced an over-the-air firmware update that gives the Tap an always-listening, hands-free option.
(Needless to say, the device might need a new name. The Echo Tap? Voice Tap? Un-Tapped? Help me out, here...)
According to Amazon, the update will be rolling out automatically in the coming weeks. Once you've got it, open the Alexa app (which will also likely need updating; check your app store) on your phone or tablet, then go to Settings. Enable the Hands-free option and you're done!
Now you can talk to Alexa without having to tap the speaker. Keep in mind, though, that hands-free mode will definitely drain the battery more quickly, as it requires the microphone to stay active at all times. You can mute the mic by holding down the play/pause button for 3 seconds (and unmute it the same way).
Amazon also said that this update adds support for Echo Spatial Perception (ESP), which "works in environments with multiple Alexa-enabled devices to ensure that the correct device responds, or does not respond, when the 'Alexa' wake word is detected."
The Tap currently sells for $130.