Coverages
Three-safety high shell
A defense that plays three safety-types deep or near-deep instead of the usual two.
A three-safety high shell puts a third safety on the field in place of a linebacker or corner, giving the defense extra range over the top and an easy way to rotate between one-high and two-high looks after the snap.
It is built to erase explosive plays and to disguise: all three defenders can drop, one can spin down into the box, or one can rob an intermediate window. The trade-off is size against the run.