Groh's 4th-down defensive scheme:
..............x...............
..............x...............
x..x.....x.x.x.x.x.....x..x
...........o.o.o.............
---------------------- <-- line to gain
........o..o...o..o.........
o..o...................o..o
Only if 4 of those o's don't have the playcall.