Complex sentence: Agreement messages (Yes votes) start to propagate from all the leaves, each leaf when completing its tasks on behalf of the transaction (becoming "ready"), and the coordinator is determined dynamically by racing agreement messages, at the place where they collide.
Split into simpler sentences: Agreement messages (Yes votes) start to propagate from all the leaves, each leaf when completing its tasks on behalf of the transaction (becoming "ready"). The coordinator is determined dynamically by racing agreement messages over the transaction tree, at the place where they collide.