Gottfried Waller testified that George Engel suggested that the signal that the armed uprising had begun would be the insertion of the German word Ruhe ("quiet" or "rest") in the Briefkasten of the Arbeiter-Zeitung. According to Waller, the anarchists at the gathering agreed that "'Ruhe' should only be inserted in the newspaper if a downright revolution had occurred."

Ruhe! was indeed published in the Arbeiter-Zeitung of May 4, the day of the Haymarket meeting. Although many strike actions and a few violent encounters occurred in the city that day, there was no evidence of "downright revolution." Still, when the police found the word Ruhe in Spies's handwriting (a reproduction of this appears in the Haymarket Digital Archive as People's Exhibit 10), the prosecution claimed to have further proof of Spies's complicity in the bombing.

Spies admitted that the manuscript was his handwriting, but he said that in preparing instructions for the typographers he had simply copied into the Briefkasten an entry submitted by a person unknown to him, and that he had no idea what the word was supposed to mean. He did not give it another thought until about 3 o'clock in the afternoon, when his associate Balthasar Rau asked him if the word had been inserted.

According to Spies, Rau explained the meaning of Ruhe, and Rau said that his knowledge was based only on rumor. Spies then summoned Adolph Fischer, who worked upstairs in the building. Fischer said that the word was "merely a signal for the boys, for those who were armed to keep their powder dry, in case they might be called upon in the next days, to fight." Spies then told Rau "it was a very silly thing" and asked him to spread the word that Ruhe! was put in by mistake, which Rau claimed he did.

Precisely why Rau, who also seems to have been the one who submitted the "Y—Montag Abend" notice of the day before, was interested in this second insertion is uncertain. He was not among the most militant anarchists. Rau fled Chicago after the Haymarket bombing, was apprehended in Omaha and charged with participating in the crime, but neither testified at the trial nor was prosecuted.

Most of the other notices in the Briefkasten are for workers' meetings (Arbeiter Versammlungen), including, further down the page, the Haymarket meeting (see next entry).