Master Pose Component
The Master Pose Component is a Blueprint callable function that enables you to set a Skinned Mesh Component Object (or Skinned Mesh Component Objects) as children to another Skinned Mesh Component Object considered to be the master. For example, you can define the Torso as the Master Pose Component, assign an animation to the Torso, then add Feet, Legs, Hands and a Head as children which will follow the animation assigned to the Torso.
Behind the scenes, the children do not use any Bone Transform Buffer and won't run any animations even if you set them on the child, it only uses the Torso's Bone Transform Buffer when rendered which makes it a very lightweight attachment system. The only Component that has to run animation is the Torso, and all attached Components will use the Torso's Bone Transform (below is an example setup where we have assigned an animation to the Torso).
Above, we use the Construction Script inside a Blueprint to set the Torso Skeletal Mesh as the Master Bone Component and the other aspects of our modular character as children.
The Set Master Pose Component function has a second parameter of the boolean type called Force Update. If Force Update is disabled, it will skip updating all runtime info if that info is the same as the Master Component. If Force Update is enabled, it will force the updating of the runtime info. This only applies to the registration process as that can be serialized, at which point it will need to refresh all runtime data.
Each aspect of our character is a Skeletal Mesh that we can change out with another Skeletal Mesh Component.
Above, we have toggled the display of the Torso and the Feet (we could change these out with different Skeletal Meshes that follow the same Skeletal hierarchy).
One issue to consider however, is that when using Master Pose Component, although it does reduce the game thread cost, it does not reduce the render cost. You will still render the same number of Components separately, keeping in mind that additional sections per Component will incur more draw calls.
There is also the limitation that any children of the Master Bone has to be a subset with the exact matching structure, you cannot have any other extra joints or skip any joints. Since there are no Bone Buffer data for extra joints, it will render using the reference pose. Also, you cannot run any other animations or physics on any children.