Bone Translation
Bone translation is sometimes used to create squash/stretch in bone chains.
By default, Power IK does not support translation of bones. If bone translation is being animated, "AllowBoneTranslation" must be checked on in the Solver Settings to tell Power IK to re-evaluate the length of bones and chains based on the incoming input pose.
When "AllowBoneTranslation" is set to true, Power IK will not try to retain the original bone lengths, but will only consider the input bone lengths.