The IK Constraint Presets makes it possible to create alternative reusable constraints sets for the same Rig. This can be used for example to model an age dependent mobility for different characters using the same skeleton
In Blender you can use the IK Constraints to model the overall mobility of your Rig. Sometimes it may be necessary to reuse the same Skeleton definition for different characters with different mobility constraints. In that case IK Constraint Presets can be a nice time saver for making quick changes or even for testing different IK configurations on the same character without loosing previous settings.
This is the list of already defined Constraints. Note that this list is stored outside of your Blend file. So all stored Presets remain available when you create a new Blend file. By Clicking on the Plus Icon on the right side, you store the current IK settings of your Rig into a new Preset.
Propagate to FK
Blender allows to define Constraints for IK and FK on the same Rig. When you enable this option while recalling a preset, then the IK Settings of the Preset will be propagated also to the FK Constraints. So your FK Rig gets the exact same constraints as your IK Rig. Hence rotating Bones manually will be constrained exactly in the same way as the IK Solver is constrained.
The last selected Preset is indicated in the Row just above the IK Solver selection field. You can delete the last selected Preset permanently by clicking on the large ‘-‘ icon on the right side of this row. Beware: The deletion is permanent and will be made instantly.