>> […] or very disasterously -- becomes impossible to spend either bitcoins of sw2x-bitcoins because of the chain overlap until the coinbase transactions start

>> […] or very disasterously -- becomes impossible to spend either bitcoins of sw2x-bitcoins because of the chain overlap until the coinbase transactions start to mature. > But I don’t see how it relates to the part I quoted. My explanation covered the "impossible to spend either bitcoins [or] sw2x-bitcoins" -- the replay means that attempts to spend one spend the other as well. Forcing a split transaction pair (as I describe above) is really the only way to ensure that your BTC transaction won't spend your SBT as well. Is it the bit about the coinbase transactions? The coinbase is the most reliable way of getting transactions valid only on one side of the fork; the mining rewards necessarily only apply to the chain on which they were mined. They aren't spendable until 100 blocks after they are rewarded, and only after they mix in sufficiently with other transactions can they be used to differentiate general transactions -- any transaction that descends from a coinbase transaction is also specific to the chain fork. This is more reliable than a timing/sybil attack to try to force the split transaction pair described above.