http://www.youtube.com/watch?v=B01nhqJizTc
Most of these "winners" are lunging stabs, but the rally starting at :50 you can see how Fed uses a slice/top spin variety with his back hand to keep his opponent moving horizontally and vertically in the court. Fed slices to his opponents forehand a few times, but very few players can hit an offensive shot off of Fed's offensive slice because the ball stays low. This is how Fed works to get an advantage in a rally very often.
Nadal hits with so much top spin already, he can get the arch on the ball to keep it in with decent pace, which makes Fed's slice less effective against Nadal. Clay also makes the slice sit up more, which is why Fed has a lopsided record against Rafa on clay. On a faster hard court, especially the indoor London courts the WTFs were held, the ball skids and stays extremely low and is too tough even for Nadal to handle. That's why Fed owned Nadal on that court two years in a row. The Miami hard courts are more grainy so they play more like clay, which is why Fed lost in 2011 to Nadal
Other players cannot handle the variety of a low slice that they must step into the court and hit a neutral ball and then a top spin ball they have to move back on without making an unforced error or leaving a ball Fed can be offensive with.
If you watch Nole def Fed in the AO 2011 SF, you'll see that Fed gets ahead in a set by playing a lot of variety and almost junk balling Djoker. It's when Fed starts going for toe to toe power rallies that Novak gains the advantage.
Nadal and Djoker will grind, or hit the exact same ball over and over until the opponent his a "bad" shot that results in an error or a short ball. This is one of the reasons why Nadal lost to Djoker 6 times in a row - Djokers backhand is far superior so Nadal would ultimately hit a ball Djoker could be extremely offensive with.
Anyway, Federer's backhand slice, which is one of the best among today's players, is rarely if ever punished and has been part of a tactic he has used regularly to be the most dominant player of all time.