[onlydarksets]

Courtesy of Wikipedia:

This play is designed for the offensive team to feign rushing one way, then attacking the defense in the opposite direction. In a counter trey right, the center, right guard, and right tackle block left as if the play is going left. The left guard and left tackle "pull" from their positions by moving behind the other linemen and around the right corner.

The running back takes an initial feint step to the left, then cuts back to the right, receives the handoff from the quarterback, and follows behind the pulling left guard and left tackle. The left guard and left tackle will usually be blocking smaller linebackers and defensive backs downfield--this mismatch favors the offense. The counter trey requires quick, athletic linemen for good execution.

Many teams have run this play, but it first became well-known when run by the Washington Redskins in the 1980s.