The change has been in the works for about a year and will, hopefully, be released in 0.10 within the next month or two. It affects the way the best chain is discovered, downloaded and verified.
This mode of operation has been known for years, but it took a long time to implement.
The code works by replacing the single-peer blocks download by a single-peer headers download (which typically takes seconds/minutes) and verification, and simultaneously fetching blocks along the best known headers chain from all peers that are known to have the relevant blocks. Downloading is constrained to a moving window to avoid unbounded unordering of blocks on disk (which would interfere with pruning later).
At the protocol level, it increases the minimally supported version for peers to 31800 (corresponding to bitcoin v3.18, released in December 2010) as earlier versions did not support the getheaders P2P message.
See the related discussion on Reddit and Bitcointalk.
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