Tanking is the downside counterpart to ripping. It describes speed and persistence rather than magnitude, and it usually implies the decline is ongoing rather than finished.
The behavioural note is that declines move faster than advances, so the decisions made during one are made under more time pressure and more arousal. That is why exit rules set in advance matter more on the way down. See panic-selling and falling-knife.
Related: ripping, panic-selling, falling-knife, capitulation