For offensive linemen, technique on running plays is much different than technique on passing plays. On passing plays, offensive linemen aren't allowed past the line of scrimmage, and drop back to protect the quarterback. But on run plays, offensive linemen fire forward in an attempt to move defenders downfield.
On occasion a lineman, often times a guard, will "pull." Pulling means that instead of firing out straight forward, a lineman will move laterally across the line of scrimmage behind the other offensive linemen and cross the line at a entry point. For example, a left guard could pull to the right and cross the line of scrimmage in between the right guard and the right tackle.
There are a few reasons why teams choose to do this. One of them is to add more blockers at the point of attack. If a run is headed to the right, there is usually a center, guard and tackle on the right side of the line, plus potentially a fullback or tight end. Pulling another player adds more bodies to help clear the way.
Pulling a player can also confuse the defense. Instead of being blocked by the offensive lineman lined up directly in front of them, a defender is attacked by a lineman from the other side of the field, which can sometimes catch the defender of guard.