Blue-shirting: This scheme was originated by New Mexico State but has not been practiced widely around the nation. Here’s how it works: Officially, a player arrives in the summer as a walk-on. Once football practice begins, he’s awarded a scholarship. The school is allowed to count the scholarship forward, but the player can play immediately.
There’s a big catch: The student-athlete may not have been recruited, as defined by NCAA bylaws. That means no official visit to campus, no in-home visits from coaches, no signed National Letter of Intent or athletic aid.
Only a handful of players, if any, would meet that criteria.
http://knoxblogs.com/evanseleven/2013/11/19/tom-luginbill-vols/