You can disregard the "bullshit I made up that sort of makes sense" comments.

A big part of the character animations in this movie is a software technology that warps body part on body part interactions instead of clipping the flesh through itself. Another aspect of that engine is handing a lot of the natural motion of soft tissue and hair as the skeleton is animated. All the bouncing of the braids, and the slight boob jiggles, is largely automated by a very, very clever piece of software. A lot of it has definitively been touched up by the animators, but most of it just happens. You can see the flesh displacement 'go wrong' at the start of this GIF on her right arm, where it pushes her bicep up to an unnatural extent further up than her forearm is actually pushing it. In this particular instance it happened because the fabric folds created by the bend pressed against the bicep rather than being further compressed like what you see at the end of the scene (particularly for the fold on her left arm). This software was first used really extensively with Tangled.

TL;DR: Nobody animated that. They moved the skeleton, and the program they work in added the boob jiggle.