The agreement between the young Tarentines and Hannibal was as follows: Hannibal on approaching the city on it eastern side, which lies towards the interior, was to advance towards the Temenid gate and light a fire on the tomb, called by some that of Hyacinthus, by others that of Apollo Hyacinthus.
Tragiscus, when he saw this signal, was to signal back by fire from within the town.
This having been done, Hannibal was to put out the fire and march only slowly in the direction of the gate.
Agreeably to these arrangements, the young men having traversed the inhabited portion of the city reached the cemetery.
For all the eastern part of the Tarentum is full of tombs, since their dead are still buried within the walls owing to a certain ancient oracle, the god, it is said, gate responded to the Tarentines that they would fare better and more prosperously if they made their dwelling-place with the majority.
Thinking, then, that according to the oracle they would be best off if they had the departed also inside the wall, the Tarentines up to this day bury their dead within the gates.
The young men on reaching the tomb of Pythionicus stopped and awaited the event.
When Hannibal drew near and did as agreed, Nicon, Tragiscus, and their companions as soon as they saw the fire felt their courage refreshed, and when they had exhibited their own torch and saw that of Hannibal go out again, they ran at full speed to the gate
wishing to arrive in time to surprise and kill the guards of the gate-tower, it having been agreed that the Carthaginians were to advance at an easy pace.
All went well, and on the guards being surprised, some of the conspirators busied themselves with putting them to the sword, while others were cutting through the bolts.
Very soon the gates were thrown open, and at the proper time Hannibal and his force arrived, having marched at such a pace as ensured that no attention was called to his advance until he reached the city.
Walbank Commentary