Kevin Sumlin won the Heisman Trophy. Even back up QBs under Sumlin put up better stats than most teams' starters. Career stats for Sumlin's QBs who threw at least 40 times.
1546/2229 - 19,217yds - 155td - 160.6rt (Case Keenum)
595/863 - 7,820yds - 63td - 164.1rt (Johnny Manziel)
201/345 - 2,641yds - 24td - 137.4rt (David Piland)
230/343 - 2,832yds - 24td - 154.8rt (Kenny Hill)
118/192 - 1,322yds - 16td - 139.5rt (Kyle Allen)
103/148 - 1,205yds - 11td - 161.2rt (Cotton Turner)
31/49 - 359yds - 2td - 134.2rt (Jamiell Showers)
27/48 - 335yds - 2td - 128.6rt (Matt Joeckel)
30/42 - 424yds - 3td - 175.0rt (Terrance Broadway)
I don't know what Sumlin does, but he makes it very easy for his QBs to succeed.