Animators call it follow-through: loose parts drag behind, carry past a stop and settle late. On Brave, Pixar's hair team called Merida's curls "an important extension of her character". But cartoon characters stop far harder than people, 10 times gravity or more, and at that stop simulated curls unwound into long, nearly straight strings. Stiffer springs kept the coils but made the hair wire-like; lighter hair floated, almost like underwater hair. Pixar's own paper says it plainly: the unwinding is physically correct. So they broke physics on purpose. A core spring runs down the middle of each curl, slack when the curl squeezes and pulling harder the further it stretches, with stiff roots and free tips, so her hair still drags behind, carries past the stop and settles, still curled. In the team's words, the tools let them "direct the hair's motion as the story demanded".

The figures and curls here are our own drawings, not images from the film.

Which movie trick should we take apart next?

Sources: Iben, Meyer, Petrovic, Soares, Anderson and Witkin, "Artistic Simulation of Curly Hair", Pixar Technical Memo 12-03b; Soares, Raja, Hurrey and Iben, "Curls Gone Wild: Hair Simulation in Brave" (Pixar), SIGGRAPH 2012 Talks; John Lasseter, "Principles of Traditional Animation Applied to 3D Computer Animation", SIGGRAPH 1987.
Sounds: our own, made in code (earshot, CC0).