Tanking in general is a term that is used when the ranker intentionally speeds up to claim top-1 position on current top ladder, sacrificing their growth for the growth of other rankers.
Whenever the person would claim top-1 position on the ladder, they no longer gain any power and vinegar, but they still get points every second and thus can become a shield for other rankers. The growing distance between tanking ranker and other rankers allows them to maintain steady growth of power and hold their upgrades longer, effectively speeding up the ladder playthrough.
Tank is a ranker that is similar to Frontrunner in general playstyle but is willing to sacrifice their growth and take early top-1 position to become a shield and maintain steady growth of other rankers.
player_requirement for the ladder is met.grape_count and extended time at top-1 spot.