Contain is a system that is based on the idea of "vectoring" an opposing player into an area of the defense's choosing. The OLBs are responsible for herding running plays to the center of the field, where the MLB makes the tackles. It's a pretty complex concept, but one that is deadly when performed well.
The theory behind the system goes like this. No runner is allowed to go out of bounds. This means that he can only take a finite number of approaches towards the endzone. In zone or man coverages, we take chances with one-on-one tackles, but what if we could change the dynamics so that the lane the opposing player takes always leads towards the center of the field? In other words, back towards ten other defensive players?
The DTs must be big, and able to hold the center of the line (not penetrate). Because the opposition will be forced to run the center as the game goes on, the DTs are critical for stopping the run. They are also responsible for protecting the MLB, who will take out any runners being vectored from the edges. For this reason, the DTs ignore most pass situations and simply hold the center.
The OLBs have the strangest job. It involves not tackling an opposing player unless that player takes a cut back towards the center of the field. This means that the OLB must run alongside the runner as the runner heads towards a sideline to keep him from reaching the edge and tearing down the field along that sideline. When the runner realizes he can't get past the defender, he'll have to cut back. When he does so, he faces the other ten defenders (heading towards him) as well as the "container", who is now allowed to make the tackle.
