Chain drooping

[fossyant]

Another vote for Freehub !

 

[ColinJ]

That is definitely my opinion too.

 

[Once a Wheeler]

Did no one else watch the video?

When you turn the pedals backwards like that the rear wheel shouldn't move or at worst may gradually start to rotate. The problem is in your freehub. It could be either sticking or the bearings are shot. Not all freehubs are serviceable.

Agree with Mr Celine. It could be a damaged pawl or pawl spring in the free hub mechanism. A joy to cure for budding watchmakers but normal people buy a new free hub. All the best with it.