What is "gain line"

n. (context rugby English) an imaginary line running across the pitch perpendicular to the touchline and passing through the breakdown or set piece. The subsequent phase is said to have passed "over the gain line" if the gain line at the subsequent breakdown is in front of the last, or "behind the gain line" if it is behind.

The gain line, in rugby union, is an imaginary line (parallel to the halfway line) drawn across the pitch at the point where there is a breakdown in open play, such as a ruck, maul or scrum. Advancing across the gain line represents a gain in territory. Playing "beyond the gain line" is a common phrase coaches use to teach their players the fundamental goal of rugby (gaining space while maintaining possession of the ball).