The IK Handle Settings appear. See Skeleton > IK Handle Tool.
A spring IK handle that starts and ends at the joints you selected appears on the joint chain.
To reset the joint rest pose for your spring IK handle
If the spring IK handle is keyframed, then you need to first disable the handle before you can reposition the joints of its spring IK joint chain. See Disable or enable IK handles.
The current pose of the spring IK handle’s joint chain is now its new rest pose.