Here is a great clip showing how much potential Alexander has. This is a goalline pass drill. The offensive player gets the route call, and the defensive back is isolated one on one with no help from a pass rush. Alexander is going against Cam Phillips, who has looked dynamic based on the little bit of video that has trickled out of Blacksburg.
Phillips gets a free release to the outside on a fade route.
Let's go 1 on 1 @Adoniskaaaaa vs. @camphil5 To be continued ... CC: @RonyellWhitaker #Hokies pic.twitter.com/CLH3mByTzC— VT Football (@VT_Football) April 8, 2017
Alexander stays square on Phillips faint to the slant and then turns to run with Phillips. As he runs, he turns his hips back so he is facing the ball. Unlike many VT DBs, who didn't have the athleticism to find the football, Alexander maintains contact with the receiver while being inundated between the receiver and the ball. Any throw that isn't right at the back pylon, Alexander is in perfect position to make a play on the ball.
This also shows that there are things Alexander can work on. He doesn't get a chuck at the line of scrimmage, so Phillips has an initial advantage. It also isn't a great throw, so you hope that Alexander could maintain contact if the throw was leading Phillips more. Still, this is a tremendous repetition.

Comments
Nice play on the ball! Can anyone tell who made the throw?
No one did. It was incomplete.
Looks like Jackson. Not Bush, as it is a right-throwing QB.
Good eye, thank you for answering what I meant.
French- this was clearly designed to be a back shoulder throw. Although not perfect, it was a pretty damn good ball. Yes, it is impressive that Adonis maintained good position for a downfield throw. More impressive is his ability to stop on a DIME and defend a back shoulder throw. There is a reason why back shoulder throws are so hard to defend and that's because most CBs can't do what Adonis just did.
Very likely yes. But we don't know if Alexander was merely reacting, or if he was playing a shallower inside technique. If it was the latter, and the O has a leading fade called, then playing that technique may not have played out so well.
Either way, his ability to get turned so early and still be on Phillips hip is very impressive. He is a terrific athlete, and even when he has had some technique issues, he can make spectacular plays.