I've looked at it a zillion times and broken it down on here using gifs, and I realize people will have different opinions, but this is what I strongly believe. The play is a fairly routine pass by Brady who puts the ball in the right area; if you want to get down to milimeters, that's a different story. It's a pass that gets caught about 99% of the time.
The play is essentially an optical illusion from the broadcast angle; it appears as though Welker has a very difficult catch because the ball is off target, causing him to whip lash back towards it. In reality, if you slow it down and look at the still images, he has a certain catch and is turning to gain more yards upfield. The problem is he mis-times the pass, closes too quickly. and doesn't assure the catch before turning.
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The problem is that he closes his hands to quickly, right on the nose of the ball.
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As for the optical illusion, I referred to...take a look at Welker's body as the ball comes out. Within a fraction of a second, he realizes it's dropped (because it's punched off his closed hands). He immediately reaches his arms towards the ball in hopes of recovering it, and his leg whips around to try to change direction towards the loose ball. That's why it appears, especially from the broadcast angle, that Welker had to whip his body towards the ball in an attempt to make a tough catch. In reality, the unnatural body movement is a second attempt to reposition and catch.
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