There's a difference between big possession receivers and skill receivers coming into the league, the smaller guys have to learn how to make separation. The bigger guys like blackmon and hurns just use their body which they've done all their lives to get open.  The thing is, once the smaller guys like shorts and lee learn how to do it, they turn into players like viktor cruz and desean jackson.  It also helps that they're placed in positions like the slot making it easier for them to get the early separation they need. 

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