As a law student I was introduced to this criterion by my professors, when asking which political speech is protected and which not. “Clear and present danger” test is used for that. If, for example, I give a speech against the government this is protected, however if this speech clearly incites a lawless action, advocating use of force, then it is illegal.
Most law professors will go as far as to this point. They will not elaborate on this criterion. Its history however is exciting. The test was introduced by Justice Holmes in 1919 in Schenck vs U.S. Schenck came in front of the Supreme Court claiming protection under the 1st Amendment. He printed leaflets against the government peacefully suggesting a change, using tough language. According to the court clear and present danger existed and Schenck was not protected under freedom of expression. The test was proven to be a trap in practice. When people needed the protection of 1st Amendment, Justices equated words with acts against the government and refused the protection.
In 1969 the criterion changed: all advocacy is protected, except for that directed explicitly to producing, and likely to produce, imminent lawless action.