Never used it on an Xperia since, well, I've never owned one... But I had that feature on an old cordless home phone and what it does is just that... slows down the incoming audio. The side effect, of course, is that it creates a lag between the real 'end of the message' and the actual time you hear it. It's great when you have an answering machine that's reading out a contact number or you just want to hear information and give you time to write it down. But for a conversation, it might create an uncomfortable number of silences hehe.