Third-Partying

Third-partying is jumping into a fight between two other players or teams once they have already damaged each other and used up resources. The third party arrives fresh, with full health and shield, and cleans up both weakened sides.

It is a legitimate and very common tactic rather than an exploit, and reacting to it is a core skill. Gunfire and building noise draw nearby players, so experienced opponents finish fights fast, reset their shield, and reposition before someone else shows up to do the same to them.