Not sure what you meant.

The first flush in spring occurs when the old wood breaks dormancy, leafs out and blooms.

The second flush on rebloomers occurs when the brand new 2025 stems attain sufficient height to trigger bud set and immediate blooming in late summer/fall 2025. So there is no such thing as deadheading of a brand new stem since those stems have not bloomed.

In addition to luis' comments, I would say that it depends on the cultivar as well. Some varieties just have more of a propensity to bloom on new growth and others less so or so late in the season as to be more of an afterthought than an actual "flush".

New Wood Aside, I thought that when deadheading the spent bloom, the side shoots below is part of the whole reblooming trait

Regardless of being old wood only or new wood bloomers, flowering on macrophyllas only occurs at the terminal ends of the stems/branches. Deadheading or the removal of spent flowers does not and cannot influence any rebloom. They just don't work like that.

The stem endings typically do not rebloom. Each stem ending with a bud will produce a bloom once per year and that is it. To get a second flush, you need for that to happen with a different stem(s) that has not bloomed yet; in the case of the second flush, it is the brand new stems that started growing from the base in 2025.

I was under the impression that touted abilities was with the spent tip removed - the side buds would have a higher-earlier potential development of stem:bloom in the same growing season.

Deadheading does not encourage side shoots to bloom. At least not this season. It is quite possible if left intact and unpruned, the side shoots could produce flowers next season at their terminal ends.

Otherwise, any reblooming features in those cultivars that are touted as 'reblooming' is generated by the new (current season's) growth maturing to the point they will flower. But only at their terminal ends :-))

But Proven Winners been claiming that its new macrophylla hydrangeas such as Sky View and Can Do have the ability to "produce new flowers along the length of the stems."

That is not a valid claim. There is nothing in the patent info that would indicate these hydrangeas possess this very unique quality and none of the accompanying photos show this to be the case. I believe that claim is based on the new growth, which appears freely and gives the appearance of continual bloom but again, flowers only at the terminal ends of the stems, and not emerging along the full length of a bare stem, which that statement implies.

FWIW, Proven Winners is not the plant breeder but only a marketing arm. And both examples provided are intentional crosses of H. macrophylla and H. serrata.

I don't have anything to add regarding macrophyllas, but my H. quercifolia Gatsby Moon has decided to re-bloom and has at least four developing flower buds. I don't know how unusual this is, but I've not noticed it before.

Oakleafs are not rebloomers but can be fooled by the weather into prematurely opening the spring 2026 dormant flower buds now instead. The blooming stem will then not bloom in spring so enjoy the premature blooms as best as you can. I once winter protected the shrub to lengthen the life of one of those premature blooms! Hee, hee, hee.