It’s not that weird. It’s just a very primitive form of “jiggle bones” (as they are sometimes known).

Basically, for 3D animation to work, you have a model, which is literally like a statue of a character. You then give that statue bones, like a literal skeleton inside their body, that tells the model how to bend so you can pose it for animation.

You can define some of those bones as “jiggle bones” which means that instead of being rigid, they have a little bit of sway to them. Modern jiggle bones are effected by physics, and get used for hair (particularly ponytails), clothing, and anything else that needs to… well, jiggle. It helps character movement look more natural and since it’s physics-based, it cuts down on animation data.

In Sonic Adventure 2, a lot of pieces on a character have jiggle bone definitions. Almost every character that has large, visible ears has them defined as a jiggle bone, in addition to Sonic’s quills, Knuckles’ dreads, and… Rouge’s breasts.

When you watch your favorite VTubers stream, you will notice something subtle yet powerful, the way their hair sways, earrings dangle, or a cape gently ripples when they move. This isn’t just “extra flair.” It’s a deliberate piece of rigging magic called secondary motion, and it can transform a flat, mechanical VTuber avatar into one that feels alive and dynamic. If you’ve ever wondered how to…