The basic premise of this play is to attack deep coverage with the threat of four people running vertical along four evenly-spaced landmarks across the field (2 hash marks, 2 sets of numbers). In general, this allows the offense to outnumber deep coverage. Four receivers running deep outnumber two safeties and stretch the area of responsibility].
It is great against any single safety look, particularly Cover 3, because of a two-on-one situation against the Free Safety. As can be seen from the 2 clips below, it is just as good against a 2 high safety look.
The adjustment is with the backside receiver who is lined up on the inside. This receiver runs what is called a "seam read". As he releases vertical he will attack the safety over the top of him. At about 7 yards he is going to determine if the safety is still on the hash, inside, or coming down towards the line of scrimmage.
If the safety is turning outside or remaining on the hash, he will bend his vertical route into the middle so as to split the two safeties and find the soft underbelly of the defense. During the drop, the QB will be looking at the locked seam on the opposite side. If after the 1st two steps of his drop, the locked seam has not won "route side" space, he will move his eyes to the "seam read" side. At this time he has hit the last step of his drop and he will be able to anticipate whether the seam read is running a post, dig, or seam route. He will hitch up and deliver the ball to the spot.
In watching Pat's eyes in the second clip, it appears this was a 4 vertical call with the front-side tagged as a post as opposed to the receiver making the sight adjustment.
I removed my boring analysis from the clips but slowed them down so you can see each receiver.