Sorry if you know this already, but how it works for me is...
When I get a text, it asks something to the effect of "You have a new text. Read it or ignore." I'll say "read it" and it reads the message. When finished it asks "reply or done." I say "reply" and it asks "what do you want to say?" So I say what I want to reply and say "send" and off it goes.
I'll admit, it integrates so well I don't know if Uconnect is doing it or if it's Cortona. Doesn't matter to me as long as someone is.
One cool trick, and I found this by searching interweb so I can't take credit... When you pair your phone and it downloads your address book it adds an entry for Cortana. So you can press the phone button, say "call Cortana" and it opens a pass through to her. You can then do any Cortana trick you want. I use it to start a text conversation from scratch without getting a text first. I.e. "Call Cortana... text Darron Smith..."