A 68-year-old man with a 20-year history of hypertension is examined before elective hernia repair. His pulse is regular at 72 beats per minute. With the bell of the stethoscope at the apex and the man rolled onto his left side, a soft low-pitched extra sound is heard immediately before the impulse felt over the carotid artery. A gentle outward movement of the apex is felt at the same moment. Nothing is audible after the second heart sound, which splits normally on inspiration.
This man's extra sound is produced during which phase of the cardiac cycle?