Are you talking about the mic icon on the app remote control screen, located towards the bottom right?
That icon just toggles using the phone mic as a receiver, basically the same as Live Listen on iPhone. That sound gets streamed directly to the HA. I’m not sure it has anything to do with the upcoming (or new) functionality update for 2 way streaming. Maybe, and hopefully, it was a placeholder and it will be for toggling the voice input mic though?
@z10user2 I do agree it’s a little funny; 80% of the time I will want handsfree. But has cvkemp mentioned, there are scenarios where you want to restrict the amount of sound being passed over to the other person on the phone.