It was a preconcerted signal.

The next instant he stabbed the captain to the heart, while each one of the galley-slaves killed the soldier nearest him; then, rushing below, they surprised and overpowered the rest of the troops, and put them all to death.
Coming again upon deck, David Gwynn descried the fourth galley of the squadron, called the Royal, commanded by Commodore Medrado in person, bearing down upon them, before the wind.

It was obvious that the Vasana was already an object of suspicion.
"Comrades," said Gwynn, "God has given us liberty, and by our courage we must prove ourselves worthy of the boon." As he spoke there came a broadside from the galley Royal which killed nine of his crew.

David, nothing daunted; laid his ship close alongside of the Royal, with such a shock that the timbers quivered again.

Then at the head of his liberated slaves, now thoroughly armed, he dashed on board the galley, and, after a furious conflict, in which he was assisted by the slaves of the Royal, succeeded in mastering the vessel, and putting all the Spanish soldiers to death.