I think the segments that are on the Edge should be updated whenever you sync, over the air or plugged in. It's a complete palaver to have to disable and then re-enable Strava Segments to get your PB's or KOM's updated on a segment and certainly can't be described as anything like "Live"
I've emailed Garmin Support and hopefully they will tell me that I'm a complete idiot and there is a simple way to do this.