The app comes in two parts. You download the K-Y Yours app, and then invite your partner to download the K-Y Mine app either over SMS or email. Both are free (thank Christ!), and once your partner accepts your request you can begin "flirting" with eachother.
The app is a private IM app between you and your partner. Except instead of sending the messages you write, the app translate them into sexy-talk.
To demonstrate, I convinced Kotaku editor Mark Serrels to be my sexy talk partner (*I was the dude). Here are some of the messages we sent and the versions received:
I sent: It's been a long day. Can't wait for the weekend. He received: I'm going to pamper you later.
He sent: I'm going to eat some chicken and shit. I received: I'm yours for the taking.
I sent: I once ate a whole large pizza and fries plus dessert. I felt like vomiting afterwards. He received: I'm picturing you in your lingerie.
So the app doesn't so much "translate" your messages as just sends completely bullshit stock phrases that are "sexy". Which makes it completely useless as an IM app because you can't actually have a proper conversation with someone. It's all about sexting, whether you want it or not. And there's no rhyme or reason to the type of messages sent either.
Our advice: Don't download this app. If you want to enjoy Valentine's Day (or any other day) as a happy couple, this is not going to help your relationship.