For each block, an LLMQ of a few hundred masternodes (300-400) is selected and each participating member signs the first block that it sees extending the active chain at the current block height. If enough members (at least 240) see the same block as the first block, they will be able to create a
clsig message and propagate it to all nodes in the network.
If a valid
clsig message is received by a node, it must reject all blocks (and any descendants) at the same height that do not match the block specified in the
clsig message. This makes the decision on the active chain quick, easy and unambiguous. It also makes reorganizations below this block impossible.
With LLMQ-based InstantSend, a ChainLock is only attempted once all transactions in the block are locked via InstantSend. If a block contains unlocked transactions, retroactive InstantSend locks are established prior to a ChainLock.