If you press the INFO button, for some programs there are two new additional options: "Resume" and "Restart show" (in addition to the existing options "Record show", "Record series", etc). "Resume" appears to be the same as pressing the BACK button; it closes the Information screen.
"Restart show" does what it says - it jumps you back to the beginning of the program.
When you watch a "restarted" show, it functions much the same as an On Demand title. (I'm going to start thinking of as "Live On Demand"). As others have mentioned, FFWD/SKIP is disabled. You also have a limited time to watch the show (e.g. It says "You have until 8:00PM to finish watching this program.")
Programs that can be restarted are marked in the program Guide with a "circular arrow" icon. Also, if you tune in a live program that can be restarted, there is a new "Restart Show" indicator in the program information banner.
I'm in the habit of recording all my favorite series, but even so sometimes I do find myself channel surfing live TV, when the ability to restart would be very useful.