Hi sevenofone,
What he is suggesting is, to move ALL elements of a single hierarchy (e.g., the right leg) in the same way, and store it in a Single Keyframe. It is an idea to fix a problem. The “mechanics” behind this procedure is, that each Joint has a Parent. This parent is the “world” for each child. If everything is moved, the Motion-Clip will affect differently. At least that is how I understand this. This works, because the joints (except the top one of the right-leg-group) has no position keyframes in the Motion-Clip. Hence they have no priority issues here. Rotations will not work here in this example as the joints have all rotational keyframes.
It assumes as well that the problem with the rigg is for all Mixamo captures of this rig the same, and that the Motion Clips in the Motion Layer will then work fine.
All joints of that hierarchy needs to be selected and keyframes at once. See below, clip is one minute long:
https://www.amazon.com/clouddrive/share/NrYtYWLSr9mJXTC51eEwsThkt6yYAEAVO3EyZ8xoisP
A downside of this is, that the manual keyframes and the Clip are not connected, any change in one or the other will destroy the solution. This needs to be kept in mind.
Initially I had posted a link with additional information about the Animation-Layer, but for now that would de-focus from this series, and might confuse right now more than it helps. Note the difference between Motion- and Animation-Layer (relative/absolute) for future explorations. For now, I hope this post cleared the question and your can enjoy this series.
All the best
