What is 'fishtailing' in a tank truck context?

Fishtailing is when the rear of a vehicle (especially a trailer) swings side to side in an uncontrolled or semi-controlled motion, often initiated by surge in a tank, sudden steering input, brake application during a turn, or slick road conditions. Once fishtailing starts in a tank vehicle, the surge of liquid amplifies each swing in the way rearward amplification works in multi-trailer combinations. Recovery requires releasing the brake (if applied), easing off acceleration, and steering smoothly to track straight rather than counter-steering aggressively. Prevention is far better than cure: smooth inputs, conservative speed, and large following distances reduce the conditions that trigger fishtailing in the first place.