The dramatic tension is created through a series of descriptive details, some of them very subtle, within the setting of the scene as Atticus sits alone in the pool of light provided by the bare bulb of his reading lamp. The men come at Atticus out of the shadows and speak in low voices. Their hats are pulled low over their faces. The smell of whiskey is noted; some, at least, have been drinking. Heck Tate, we learn, will not be coming to help; Atticus will face them alone.
The tension increases when Jem, Scout, and Dill suddenly appear. Atticus' fear shows in his eyes and his deliberate movements:
Atticus got up from his chair, but he was moving slowly, like an old man. He put the newspaper down very carefully, adjusting its creases with lingering fingers. They were trembling a little.
Tension mounts when Jem refuses to take the children and go home. Atticus tells him twice; then, indicating his desperation, Atticus seems to plead, "Son, I said go home." The situation becomes more volatile as one of the mob suddenly grabs Jem by the collar and pulls him off his feet, threatening harm.
The tension, at least for the reader, is momentarily relieved with a bit of humor as Scout kicks Jem's attacker:
Barefooted, I was surprised to see him fall back in real pain. I intended to kick his shin, but aimed too high.
Matters become serious again, quickly, when Atticus is given a warning: He has fifteen seconds to remove his children to safety. What follows is Scout's innocent conversation with Mr. Cunningham. When Cunningham leans down, takes her by the shoulders, and speaks to her directly and gently (calling her "little lady"), the dramatic tension is resolved. Atticus is no longer facing a mindless, anonymous lynch mob.