Well--I am no Phil but here goes...
A U-Turn is a new twist this season that we assume is replacing the Yield. A team may, only ONCE per race, choose to U-Turn a trailing team after a Detour. That team will then have to go back and perform the other (previously not chosen) Detour.
The mechanics seem much like the Yield--either state for the cameras that you do not wish to use the U-Turn, or place a picture of the team you are U-Turning along with your own picture as U-Turners and you are done.
I like this better than the Yield as teams still have some remmnant of control over their own fate, rather than just standing by helplessly watching the sand in an hourglass go down.