From Guild Wars Wiki
The term lineback refers to using your melee characters to apply pressure to the other team's melee characters. This often slows the enemy's damage output, and discourages the enemy from overextending or going into Frenzy.
[edit] Situations where it's best to lineback
- Your party is about to wipe and you want to survive long enough to pull back into your base.
- A character dedicated to linebacking is an element of your party's defense.
- The enemy team has a powerplay.
- The enemy is a very strong pressure team.