An offset I formation in football is a variation of an I formation, but the fullback sets up behind the guard instead of behind the quarterback. A typical offset I features two wide receivers and a tight end as pictured above, but other variations exist with two tight ends or flankers.
Additionally, some more unorthodox offset I formations may feature overload line sets and wing backs.
Offset I is an old school run game formation and it excels at clock-chewing, short run game play. Most often, the fullback will serve as a powerful lead blocker, punching holes in the defenses secondary for the halfback to run through. Straight dives to the strong side are a definite strong suit of the offset I, but trap plays are just are versatile. These are plays where a lineman will pull and move down to block at the hole closest to the play. This creates some misdirection in the backfield and makes it more difficult for linebackers to read plays.
Depending on the alignment of the receivers and the tight end, offset I can be great for the short pass game. Often times, the backers in offset I will stay in the backfield to help protect the quarterback, but other times they will go out for short waggle or screen routes. Some of the most versatile plays in an offset I are the run-pass-options. Since this formation is so run heavy, it can be easy to catch the defense off guard and throw to a wide receiver on a quick in route or slant for a pick up of five or so yards.
The major disadvantage of offset I is its dependence on the run game. It is a pretty straightforward, in-your-face offense with little misdirection. Proper adjustments for defenses encountering offset I offense rely on loading the box with eight defenders and preventing gains of more than two yards per play. Though a defense may over anticipate the run game, diligent linebackers will remain prepared for quick passes or proper man defense coverage will prevent receivers from being open. The true key to stopping an offset I offense is minimizing gains, rather than stopping plays altogether.