Force is flick. Handler-marks are being forced back; no cheap up-the-line cuts. 5 drops down and covers the break-side in the endzone.
If the offensive player cuts to the open-side, 4 cuts him off on the open-side, 5 on the break-side. A call is made by 5 (e.g. "red") to let 6 know that he must now cover the break-side under.
If 4 then tries to cut to the break-side (or makes his initial cut there), 4 and 5 do the same job, with 6 poaching more to the open-side.