ASICBoost is a new method to mine bitcoins with a 20% speedup, as covered in this article:
Now, consider the following diagram:
Apparently there are two versions of AsicBoost - covert and overt. Overt AsicBoost involves changing the Version section of Chunk 1 only, while Chunk 2 remains unaltered (except for the nonce, which is still incremented). This is faster than the traditional method of incrementing the nonce, and then adding the extra nonce.
However, doesn't Overt AsicBoost reduce the search space? The current Bitcoin network difficulty is 15,546,745,765,529, which means that a miner has to iterate through approximately 2^76 hashes before a valid block header hash is found. However, the Version section of the block header is only 4 bytes (32 bits), and the nonce is also 4 bytes (32 bits). Therefore, if a miner implements the Overt AsicBoost technique, the search space is only 2^32*3^32=2^64 hashes, which is far smaller than 2^76. Wouldn't this be completely insufficient to find a valid block header hash?