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One of the great strengths of an up-tempo offensive system is its ability to limit the defense's ability to substitute. That strength becomes exponentially stronger when the offense doesn't have to substitute, either.
Playing at a quicker pace makes versatile players more valuable, and the Irish are loaded with multi-talented offensive weapons.
Regardless of what position group Theo Riddick is lumped in with next season, he's proven that he has the skill to play both running back and wide receiver.
With Riddick on the field, the Irish can spread things out and force the defense to show its hand. If Notre Dame lines up with an empty backfield and the defense counters with a nickel or dime package, the Irish can motion Riddick into a tailback position and instantly create an advantage with a running formation. Even if he stays at wideout, Riddick can be a weapon on reverses and jet sweeps.
If Riddick lines up at running back and the defense covers him with a linebacker, he can motion out wide and use his agility to get himself open against the slower defender.
George Atkinson III is also a bit of a tweener, but he's shown an explosive skill set on special teams and could fill a similar role to Riddick's on offense.
A receiver/runner hybrid will be important for Notre Dame, but the most versatile position for the Irish has to be tight end.
Tyler Eifert has field-stretching speed on a frame that can create lanes in the running game. With Eifert on the field, the Irish can very easily shift between power and spread formations without having to change personnel. He can excel in either role, whether he lines up next to an offensive tackle or flexed out wide.
Like Atkinson, Ben Koyack hasn't proven himself yet, but has a toolbox of abilities that would fit perfectly in the same role that Eifert will play.
Notre Dame is loaded with athletes, and running a fast-paced system allows Kelly to move those athletes around to take advantage of every one of their capabilities while creating mismatches for the defense.