The diction, or word choice, of Doris Lessing's description of Jerry's swim through the tunnel is suspenseful as it creates anticipation of Jerry's successful passage, and the sense of danger is certainly created through the selective use of certain words.
The sense of danger is conveyed by the use of such words as "fear" and "horror" as Jerry shakes with fear that he cannot bring himself to attempt passing under water through the tunnel, and he trembles with horror at the thought of swimming through a "long, long tunnel under the rock," under the sea. Other words such as "panicky," "frightened," "dying," "struggled on in the darkness," suffering "lapses into unconsciousness"and "groping forward" suggest the struggles of Jerry as he traverses the long passage under water.
Also, as in other passages, Lessing employs counting numbers as Jerry measures the time that it takes him to pass through the tunnel. For example, Jerry counts as he swims: "[A] hundred and fifteen, a hundred and fifteen pounded through his head," higher numbers than he has counted in his practice sessions. These high numbers create tension as the reader has known that Jerry has not counted holding his breadth this long before.
Other words and phrases that convey tension and danger are those such as "His head was swelling, his lungs cracking"; "His hands, groping forward, met nothing; and "his feet, kicking back, propelled him out to the sea." Further, when Jerry is "gasping" and "a gout of blood" passes into the sea and he knows that his nose has been bleeding, there is anxiety created in the reader that Jerry may have injured his lungs.
Clearly, Doris Lessing's diction and style of description draw the reader into the narrative as suspense and tension is created, lending much verisimilitude to the passages about Jerry's attempts to swim under water for a long stretch as well as his final attempt to pass through the tunnel as the older boys have done and complete his rite of passage.