So one the end of an opponent's turn, you cast {Mystical Tutor} and search for {Temporal Mastery} to the top of your library. Then during your draw stemp, cast it, cast any number of "copy target spell" spells you've got, then go to town. Shame it gets exiled after you use it, but this allows you to turn the game in your favor very easily. I use it in my mono blue EDH deck the turn before I combo, so if I somehow brick on the combo, I get an extra chance. Alternatively you can just use it to gain some ground. I also have {Sigil Tracer} and {Echo Mage} in there (They're part of another combo with {Turnabout}) so if I happen to do that when I have them, it's almost a {Time Stretch}.
Mystical tutor is banned in just about everything.
But not in EDH. Which is what he's talking about.
Too many people have started abusing this, Jhoira players sometimes do this just to remove the time counters on a suspended card.
And yeah, I am guilty of doing it as well, because it is too funny to say "it's a miracle!" When it happens. :)