The 4-3 under is a defensive formation with four defensive lineman, three linebackers and four secondary players where the defensive lineman are shifted away from the strong side.
The name under comes from an under shift. The opposite is an over-shift toward the strong side. By moving the 3 technique defensive lineman next to the 5 technique the defense has both the B and C gaps covered. The defense is better able to defend the point of attack on the weak side since they have shifted their line that way.
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