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When she first attended classes run by the self-help group Nxivm, Sylvie noticed multiple pictures on display of the group’s leader, Keith Raniere, known as Vanguard.

People in the classes, held in Albany, appeared to venerate Mr. Raniere, Sylvie testified during his racketeering and sex trafficking trial. She said class participants clapped, bowed, huddled, recited a “mission statement” and then said in unison “Thank you, Vanguard!”

The experience left her thinking she would never take another class, Sylvie said. Yet she did, eventually joining a clandestine subgroup within Nxivm in which she was called a “slave” and required to blindly obey a “master.” She even allowed Mr. Raniere to perform oral sex on her, believing she could not refuse.

For six weeks, the question of how Mr. Raniere persuaded so many seemingly perceptive people to let him control their lives has hung over his federal racketeering trial in Brooklyn.