When I saw the cover I presumed that the lean into the ball meant his left sided sag... but the fact the the illustration showed a DTL view made me wonder if a "feel" of leaning towards the ball may not be unhelpful...
I then edited that video and found that he did have a lowering of knees and head and upper body...mostly occuring during the backswing but maintained during downswing.
It appears to be a whole body concertina compression - as others have described elsewhere - rather than a simple head bob...IMO
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