How is suspense built up during Stephen's night excursion to the other side of the tunnel in chapter 6?

Stephen's night excursion to the other side of the tunnel  is full of suspense. Although the adventure probably didn't last  long, the meticulous description of its every detail slows its narrative pace and at the same time makes it very vivid. As a result, the reader gets engaged in the events and feels eager to find out what's going to happen next.

Sensory images and memory

At the beginning of the book, we can find that the narrator uses many sense-related images to describe his memories. One of them , for example, is when Stephen tries to describe a peculiar smell whose origin he could not determine ("It's something quite harsh and coarse. It reeks."). Other examples would be when he makes an effort to recreate the texture of the scaly belt he wore as a child ("In the tip of my fingers, though, even now, I can feel the delicious silvery serrated texture of the snake's scaliness.") or the failed garter of his sock (“I can feel it in my fingertips, as clearly as the scaliness of the snake, the hopeless bagginess of the failed garter beneath the turned-down top), or when the sound of a train makes the past rematerialise, and turns the present Close into his childhood street.

The use of these sensory images was proved to be a good method to remember things in a TV show called "Brain Games". In an episode of this show, the host asked a group of people to memorize a certain amount of items in a room. In their first try they could only remember around 3 or 4 items out of 10, but in their second try, the host told them to think of a sensory aspect of each item to see if  their score improved. To their surprise, it worked. This time they could recall around 6 or 7 of the 10 items.

The studies and experiments made in "Brain Games" show why Stephen focuses on sensory elements to retrieve his childhood memories.  As shown in this TV show, using the senses helps us remember.