Defending the deep ball has always been more challenging and technically involved than it appears.
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When physical skills are equal, as they often are in the NFL, precision and patience take over. This is where Landry failed at first, as a young safety at Georgia Tech. When a receiver beat him deep in a one-on-one practice drill, Landry decided to turn around and play the ball. That might feel like a natural reaction, but it is all wrong, because you lose track of the receiver, drift away from him and slow your pursuit, all at once.
Landry said defensive backs are always taught to never look back for the ball until they are “in phase” with a receiver – basically running next to him, hip-to-hip. Once in phase with the receiver, the defensive back should “look and lean,” Landry said. That means simultaneously turning back for the ball while slightly leaning into the receiver’s area.
The purpose of the lean is twofold: It lets the defensive back physically track the receiver while not looking at him, and also closes the spacing between the two players.
Another source
With his hand on the receiver’s hip, the DB should turn into the receiver and attempt to maintain contact with his hip. The DB’s eyes should now be focused on the receiver’s ear hole. If the DB loses contact with the hip, he must make every attempt possible to close the distance between himself and the inside hip of the receiver, while always keeping his eyes on the ear hole of the receiver’s helmet.
As the receiver turns his head to look for the ball, the ear hole disappears. The DB must then focus on the eyes of the receiver. If the eyes get bigger, the ball is coming. At this point, the DB must look for the hands of the receiver. As the hands go up to catch the ball, the DB must place his hands inside the receiver’s hands.
The DB must then turn his head and make a play on the ball. If he cannot place his hands inside the receiver’s hands, the DB must either rake down on the receiver’s arms or punch up between the hands to take away the football. If he cannot secure the football, he is in position to make the tackle.