the response to the rows command is not automatically followed by the next command, but instead, if a rows command is sent by either end in response to a hash command, the next command automatically follows

This effectively adds symmetry back, so that the 'to' end will send the next hash if it doesn't match a given hash, rather than waiting for the next hash to come back after the rows command it sends.  This saves half a round-trip on average, in this case.