Read the text here: Acts 22:22-30.
The crowd who up to this point had listened to Paul’s address, responds with fury to Paul’s assertion that he has been sent by God to the Gentiles. It parallels the response of the congregation in Nazareth to the preaching of Jesus in the synagogue (“no prophet is acceptable in his hometown”). They begin shouting him down, throwing down their coats and kicking up the dust. Think of an irate baseball coach kicking dust onto an umpire over a disagreeable call (see Spencer, p. 221). What a theatrical moment! Paul has kept all of his remarks well within the religious tradition of Judaism, but the crowd (for undisclosed reasons) will not accept the extension of God’s blessing will go to the Gentiles.
The Roman captain has no framework to understand this in-house disagreement between two Jewish factions, but the violence is enough to convince him to keep Paul in custody. He decides that Paul should be “examined by flogging.” The Romans were not known for the respect for human rights, and the prospect of beating a confession out of a prisoner would not have been uncommon at all. Flogging of suspects, particularly of someone from the lower classes, a foreigner, or slave was an accepted, legal practice of torture.
Finally as his arms are outstretched in preparation for the brutal interrogation, Paul plays his trump card. He is not only a Jew, and not only a citizen of Tarsus, but he is a Roman citizen by birth. The centurion overseeing the torture stops, and goes to the captain with this new information. As a citizen, Paul would be eligible for a trial and a flogging without proper legal preceedings would have been a serious legal breach of due process.
The flogging is abruptly canceled.
And the stage is set for Paul’s next big speech in front of the Sanhedrin.