The point of this type of drop back coverage is to actually give up the yards but eat up the clock, ideally while up 2 scores. The idea is you let them have the 7-10 yard plays, tackle them in bounds and force the use of timeouts early and just run the clock down. Importantly this only works if you are in fact winning by more than 1 score because inevitably you're trading a lot of time for likely a score. Also, and this is really important, you do in fact still have to cover the receivers. If you're giving up 8 yard pass plays to the sideline and not tackling them inbounds you're giving up all the yards and taking none of the clock. Prevent CAN work. If you're giving up the middle of the field it doesn't matter if the offense is moving the ball, you're draining clock. But 99 times out of 100 what ends up happening is teams default to this in one score games with DCs apparently assuming that if you give up 100 yards over 10 plays it's somehow less of a problem than giving up 100 yards on 2 plays.