The basic theory of the "prevent" defense was that it was near the end of the half or the game with small amount of time left and the opponent has gained possession of the ball deep in their own territory with a long way to go to pay dirt. Rather than play up close and tight and aggressive in trying to stop the offense from getting even a first down, you basically were going to prevent them from getting a big play or score and give some ground and kind of let them use up the game clock time remaining just getting short gains. The defense is baiting the offense in to taking short gains while using up their limited time remaining.
The 'bend but don't break' defense is really just a friendly way of saying a weak or soft defense that has trouble stopping the oppoent from marching down the field on them. The only other argument sometimes offered for the prevent or soft type defensive schemes are that if you force the offense to make a longer, 10 plus play drive, there is a greater likelihood the offense will make a mistake and get a penalty or make a turnover or otherwise fail to execute a given play. This then will put the offense in a disadvantaged down and distance or other situation where the defense has a significant advantage.
The biggest problem with the soft and prevent defense mindset is that you allow your opponent to have time of possession in the process. Your own offense can't be scoring if it is on the bench and doesn't have the ball while your defense is out there basically in tackling drill all game long. The more plays the defense has to face, the more fatigued it will become. Our defense was worn down by Ark State. This is something that is absolutely NOT Nebraska football. We should NEVER be worn out. We have plenty of players to substitute in and need all the playing time they can get. If the starters are tired, put in the 2s and or 3s. This is how you build depth.
I liked Bo Pelini's mindset on defense of switching up and playing tight or loose or feigning aggressive / passive throughout the games. He was always trying to ge the offense in a bad down and distance situation. He would back off and play soft or come up and 'in your face' kind of like his own personality. He was particularly good when he had Suh which of course is the key in football. The team with the best players is going to win most of the time. Scheme and strategy are nice but talent is the biggee!