Brian has talked about Va. Tech's base D briefly, but I'd like to talk about it more in-depth. The "G" defense, as it is known, was the base D for the high school team for which I was a varsity assistant for 9 years, including the last year as defensive coordinator. I say this not to try and impress you, but to let you know that I have a good knowledge of the defense.
The defense was built to stop 2-back, 1-TE formation offense (also known as "21 personnel"), especially in the run game. Refer again to the diagram of "G" against the Pro formation:
The corners are the only "pass first" players on the defense, so if the offense runs a regular run play (i.e., where the entire OL run blocks and the play is a run), the defense has 9 defenders in the box to defend the run.
The defensive alignment pre-snap looks like Cover 3 or Cover 1 (aka man-free). The base coverage of the "G" defense is "robber" coverage, in which the underneath and inside defenders play a "robber" technique. The corners start a 7 yards deep and the free starts 11 yards deep over the strong B gap. Late in the QB's cadence, the corners will slowly backpedal inside and 9 yards deep, while the free safety will creep toward the line of scrimmage from 11 to 9 yards.
The free safety will read the EMLOS (End Man on the Line Of Scrimmage) to passing strength (the side of the formation with more receivers) for run/pass. If it is a run, he is to front the ball (get in a direct vertical line with it), then play the ball inside-out, while staying inside the ILB (Backer or Mike) to which the ball is being run.
If it is a pass, he is to read the release of #2 strong. If #2 is vertical (any route where #2 hasn't broken off by 7 yards, inluding deep outs, digs, hooks, etc.) the free plays him man. If #2 goes to the flat, the free "robs" #1 to that side, mostly expecting a 12-15 yard curl route or deep in route. If #2 runs a shallow cross, the free "robs" #1 to that other side, mostly expecting a dig. In either robber situation, if #1 runs a post, the free has deep help from the corner.
The advantage of playing against a pro set is that the EMLOS and the #2 receiver are the same player: the TE.
The Rover and Whip key the EMLOS. Against the run they are primary force players. Against the pass they are to "buzz flat, run wheel," which means they sprint to take away a quick hitch, out, or slant. If the receiver in the flat turns it up on a wheel, the Rover/Whip will carry him man-to-man, allowing the corner to be an inside leverage, deep half player.
The Backer robs #3 strong and has the same rules as the free: #3 vertical = man, #3 flat = rob #1 strong, #3 shallow = rob #1 weak. If #3 stays in to pass black, the Backer will play any shallow cross or checkdown.
The Mike robs #2 weak, and his rules are similar: #2w vertical = man, #2w flat = rob #1 weak, #2w shallow = rob #1 strong. If #2 weak stays in to pass block, the Mike will drop to the weak curl zone.
I hope to write another diary discussing the weaknesses of the G defense with robber coverage.