https://dev.epicgames.com/community/learning/courses/5vL/unreal-engine-creating-and-modifying-control-rig/DnO1/unreal-engine-creating-and-modifying-control-rig-overview
https://docs.unrealengine.com/5.0/en-US/control-rig-in-unreal-engine/
Use the Construction Event to set up starting values before any solver sequence is applied.
Forward Solver - Controls move the joints. This is the sequence that runs the animation blueprint during game play.
Backwards Solver - Bones influence controls, allowing transferring animation sequences to control the rig.