A good article
https://theringer.com/watching-football-nfl-scout-guide-4182363cb930#.9tzj39uqp
Here is a good example of what the article is talking about in this nice evaluation of Gareon Conley and his use of hands. It's a make or break for a Defensive Back.
https://www.youtube.com/watch?v=yfJDXra4L14&feature=em-subs_digest (https://www.youtube.com/watch?v=yfJDXra4L14&feature=em-subs_digest)