The current Huskers run two main options; the belly option and the Speed Option. The belly option was the option perfected by Coach Osborne's teams. Its really built around the path of the fullback and quarterback. The QB reverses out, opening away from the play call. The FB takes an off-tackle path. After the fake from the QB, the FB bounces outside to lead the QB if he keeps. Its always ran out of a two back set.
The term "G" simply means "Guard" and it stems from the "G" blocking scheme where the Guard on the play side pulls and then immediately targets the end man on the line of scrimmage. The play side tight end and tackle both block down which should seal the edge. The fullback then has to read the guards block. He can then go inside or outside of the pulling guard, but has to read the guard to determine where his block needs to go. The quarterback then has the "option" to keep or pitch based on the pursuit of the unblocked linebacker or DB attempting to move up in support.
Because Martinez also spins away from the playside, he can also handoff to the Fullback with the Fullback trap. When it works, there is usually nobody within 10 yards of the fullback by the time he reaches the 2nd level of the defense. When it doesn't work, the FB can be stood up for a 2 yard loss.