Posted on Sept. 28, 2020, 10:44 p.m. by crute • Last updated on March 4, 2021, 5:13 p.m.
1 Running some sort of simulation on mesh or a part of a mesh, so its structure reacts to motion. This is usually done with a cloth or a softbody simulation. To reduce the time needed to calculate the simulation, it is oftentimes run not on the target mesh directly, but on a proxy mesh with lower resolution. The proxy mesh is "bound" to the target mesh, so the proxy movement can be transferred over to the target. This can be achieved with the Mesh Deform or Surface Deform Modifiers.
2 Using the Armature's bones to get secondary motion. This is usually achieved with the help of a plugin such as Wiggle Bones, Jiggle Bones, Spring Bones or SimBones. All of them extend the Armature System by a property for jiggling that you can turn on or off that will enable physics on the part the bones controls.
3 Animating Jiggle by hand. It requires the most work, but it is also the most precise and customisable way of achieving the desired results. This can be done by simply animating the bones, usually by offsetting part of an animation to occur later or by sculpting custom shapekeys that have the desired deformation and then activating them at the proper time.
There is an add-on for the setup, it gets quite involved without that. Currently needs an older build of 2.8. Pre RC1 https://github.com/dskjal/Rigidbody-Bone