With Curtains the only way to chat is if you're both actually there.
Mobile messaging apps are mostly designed for asynchronous communication – you can send a message, and it might be hours or days before anybody reads it. Curtains ensure that if you're messaging with somebody that you're both actually there. Both present in the moment.
There is no "send" button!
For the messaging to be truly synchronous, you shouldn't be composing a message and maybe sending it later. Every letter you type is out there and visible by both of you as you type it. Know that warm and fuzzy feeling you get when you see people "typing" on Facebook Messenger? Well, this is even better!
If the curtain is open, you know you're both there.
The curtain is trivial. If it is open, you know your chat partner is there with you. Not playing that game they always do. Not switching about between apps. Not chatting with other people. They are there with you!
No exchanging of user names or phone numbers.
Curtains Messenger is integrated with Facebook. All the people you have as friends on Facebook who also use Curtains are there available for you.
It is not perfect, but it gives you a taste of how different synchronous messaging is. Go ahead and get it :)