Here is the passage that I have read from "Sir Gawain and the Green Knight," followed by a brief synopsis. It comes from the end of Part 1, lines 417-429.
"The Green Knight upon ground girds him with care:
Bows a bit with his head, and bares his flesh:
His long lovely locks he laid over his crown,
Let the naked nape for the need be shown.
Gawain grips to his ax and gathers it aloft-
The left foot on the floor before him he set-
Brought it down deftly upon the bare neck,
That the shock of the sharp blow shivered the bones
And cut the flesh cleanly and clove it in twain,
That the blade of bright steel bit into the ground.
The head was hewn off and fell to the floor;
Many found it at their feet, as forth it rolled;
The blood gushed from the body, bright on the green,"
This passage describes the scene in which Sir Gawain cuts off the head of the Green Knight. This happens as a result of a test that the Green Knight proposes, and that Gawain accepts. It is a very descriptive passage, because of the graphic words that are used, like "cut the flesh cleanly" and "blood gushed."