Interesting article, worth a read
Some key points
The key to this change sounds simple but is a dramatic shift from NFL norms. Basically, the Saints want to play the best 11 players they can find. Size and position are of lesser importance. Former NFL scout Daniel Jeremiah, now an analyst at the NFL Network, calls the Saints' plan "the most fascinating" scheme of the 2014 season.
The idea was hatched by accident last year, when injuries to linebackers gave Ryan a dilemma: play bad linebackers or get creative with positions. Ryan went the latter route and stressed the safety position, playing as many as four safeties at once and playing three at a time in his default defensive package. In the NFL, some teams play as few as one safety and almost no team ever employs more than two.
Safeties are bigger than cornerbacks, who typically cover wide receivers, but faster than linebackers, who are built to stop a running back and take on offensive linemen. They can be 60 pounds lighter than some linebackers but 20 pounds heavier than some corners. They can cover the insanely athletic crop of tight ends now in the NFL and take on the league's rising group of tall receivers all while giving up only a little bit of speed from a cornerback.