Most attempts to "improve posture" add effort. They ask a part of you to do more.
The wave works the other way. It asks the whole of you to do less — to stop insisting on one direction so the other can arrive.
Try this: stand. Don't correct anything. Just notice — is there a place in you that's bracing to hold you up? Let it soften by a degree. Not collapse. Soften.
That small release is one phase of the wave. Engagement is the other. You don't manufacture either. You allow the turn-taking.
If this lands as a feeling and you want the full map, the book takes it further.