From, Dio's Roman History, E. Cary (tr.), v.3. p. 140-145,
Cambridge, Mass.: Harvard University Press: 1914
[27.1.1]Hence there arose turbulent factions and contentions about the court the one party demanding that it should not be convened and the other that it should. When the latter party won, because of Caesar and some others, there was another clash regarding the character of the trial. [27.2.1]Caesar himself was judge together with Lucius Caesar, for the charge against Rabirius was no ordinary one, but that of perduellio, as it was called; and they condemned him, although they had not been chosen according to precedent by the people, but by the praetor himself, which was not lawful. [27.3.1] Rabirius appealed, and would certainly have been convicted by the people also, had not Metellus Celer, who was an augur and praetor, prevented it. When nothing else would cause them to heed him and they were unconcerned by the fact that the trial had been held in a manner contrary to custom, he ran up to the Janiculum before they took any vote at all, and pulled down the military flag, so that it was no longer lawful for them to reach a decision.
[28.1.1]Now the matter of the flag is as follows. In ancient times there were many enemies dwelling near the city, and the Romans, fearing that while they were holding a centuriate assembly by centuries foes might occupy the Janiculum and attack the city, decided that not all should vote at once, but that some men under arms should always guard that position. [28.2.1] So they guarded it as long as the assembly lasted, but when this was about to be adjourned, the flag was pulled down and the guards departed; for no further business could be transacted when the post was not guarded. [28.3.1] This practice was observed only in the case of the centuriate assemblies, for these were held outside the wall and all who bore arms were obliged to attend them. Even to this day it is done as a matter of form. [28.4.1] So on that occasion, when the signal was pulled down, the assembly was adjourned and Rabirius was saved. Labienus, indeed, had the right to bring suit again, but he did not do so.