Oakland University Practice Notes
4 on 4 shell, on each pass any defender who is one pass away from the ball must touch the on-ball defender then recover to their one-pass-away help position.
Here X4 closes out on a pass to 4, X1 and X3 touch X4 then get into defensive position.
All four defenders move on the flight of the ball. The goal is to emphasize recovering to help, not to a check.