TwentyTen Header Image

Hi, can anyone explain how the below works? Why does the script detect a new header image when:
1. a singular page is displayed
2. with an attached image and
3. $image[1] >= HEADER_IMAGE_WIDTH (does this mean that the new image has a larger width than the width defined in the functions.php file?)

Thanks in advance!

<?php
					// Check if this is a post or page, if it has a thumbnail, and if it's a big one
					if ( is_singular() &&
							has_post_thumbnail( $post->ID ) &&
							( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) &&
							$image[1] >= HEADER_IMAGE_WIDTH ) :
						// Houston, we have a new header image!
						echo get_the_post_thumbnail( $post->ID, 'post-thumbnail' );
					else : ?>
						<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" />
					<?php endif; ?>