Regarding trail technique. Getting into a trail position is if option A fails. What Saleh really wants is for the CB to be hip to hip with the WR with optimal positioning being the CB's outside shoulder (shoulder closer to sideline) on top of the WR's inside shoulder. At that point look up for the ball because the CB is now controlling the WR, able to body up against him.
If the CB is unable to be hip to hip with the WR, that means he's now trailing the WR. Only then will the CB not look back for the ball, and instead play the eyes/hands of the WR. This is for vertical routes up the sideline.
This article explains the techniques a CB will use, with words from players we know (Eric Davis, Tim McDonald).
http://www.nj.com/jets/index.ssf/2014/09/the_art_of_defending_the_deep_ball_how_one_of_footballs_most_stressful_situations_is_getting_even_to.html
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.
What if a defensive back gets beat, and trails his receiver? In this situation – "out of phase" – the defensive back must look at the receiver's hands. When he sticks them out to catch the ball, the defensive back has to jab his own hand ahead to disrupt the play.
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Here's Sherman in phase, outside shoulder on top of the WR's inside shoulder. He is in proper position to look back and play the ball.
Revis in phase. In both these plays, the CB goes for the jam by turning then punching with the arm that will eventually be the outside arm (outside = closer to sideline).
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This is DJ 'out of phase'. His right arm (white sleeved) should not reach out like that. That's the wrong arm to use. Compare DJ here to what Sherman and Revis do above. Sherman/Revis don't use that upfield arm to jam at the line.
There is a problem with this 'play the hands' technique that gets CBs into trouble - when the pass is underthrown. When it's underthrown, we always think, "he should have turned around, probably could have gotten an INT". But if CBs are taught to turn around when trailing, they will risk (from the article), "
lose track of the receiver, drift away from him and slow your pursuit, all at once."
imo the issue with DJ isn't so much that he can't play with proper technique when he finds himself out of phase, although it is an issue. But the main issue is that he is rarely in phase.