Lefty is mighty enthusiastic in that clip, isn't he? He must have been hopped up on In-n-Out burgers on the day of the shoot.
Mickelson didn't invent this shot; the backwards golf shot has been part of golf trick-shot artists' routines for many, many years, and it's not a difficult shot to pull off for very skilled golfers (assuming the circumstances are right for it, of course).
An interesting nugget from the above clip is when Phil explains that he has had to use the backwards shot in competition before.
Here's another look at Phil's over-the-head shot, in which he blames the ball not going in on the grain of the green (funny Phil!):