Pretty interesting. Hope someone also writes a summary of how it works. Are they computing the tree in a worker, or have they just modified the rendering api itself to somehow be interrupt-able?
If a user input or other render - triggering event occurs partway through a render pass, React can set aside its partially completed calculations, start a new render pass, complete that and commit it, then effectively "rebase" the original changes on top of the just-applied change.
Shawn Wang has been collecting links to all the info on Concurrent React that's been released over the last couple years :