The tension in this portion of Chapter Six begins long before the chapter begins. The stories about Boo Radley stabbing his father and their mysterious solitude sets up the fear and intrigue that can be a brutal combination in young, curious children.
However, it isn't just the stories that create tension, but the gifts left in the tree. This discrepancy in the concept of Boo Radley, ultimately lures the children to the door.
In this chapter, the children create their own tension beyond that of the inherent mystery surrounding the Radleys. The boys are trying to prove their manly courage, while Scout struggles to check her fear and maintain her status in her brother's eyes, especially after Jem admonishes,
Scout, I’m tellin‘ you for the last time, shut your trap or go home—I declare to the Lord you’re gettin’ more like a girl every day! (Ch 6, 28)
Additional details like the squeaking gate and the creaking boards create classic tension as the reader can actually hear what the residents of the house must also hear.
The tension reaches a peak when the children all see the shadow; Scout, the narrator, sees it first:
Then I saw the shadow. It was the shadow of a man with a hat on. At first I thought it was a tree, but there was no wind blowing, and tree-trunks never walked.
The scene culminates with Jem having to leave his pants on the barbed wire fence to avoid capture.