It's actually very simple to explain to you why arm length matters to DTs.

Think of a reach advantage in boxing. Longer arms are a big advantage, right?

Well, a guard or tackle vs a DT has a great advantage if his reach is longer in the same way.
If that OT or OG gets his longer arms inside and grabs some pad and jersey, a DT with shorter arms is now at a disadvantage trying to shed the block and make a tackle or move past the offensive lineman.

Simple.

Conversely - a DT with longer arms than a guard or tackle has an advantage shedding or rejecting a block.

Pat Kirwin explained it in his book like this:

Quote:In his book Take Your Eye Off the Ball, Pat Kirwan likens the clash between the offensive and defensive lines on each play to “a series of adjacent bar fights.” If you control the chest, you control the man; if you control the man, you control the gap.

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