WordPress at a glance
filter is not described

comment_form_defaults filter-hook . WP 3.0.0

Filters the comment form default arguments.

Use 'comment_form_default_fields' to filter the comment fields.

Usage

add_filter( 'comment_form_defaults', 'filter_function_name_4551' );
function filter_function_name_4551( $defaults ){
	// filter...

	return $defaults;
}
$defaults(array)
The default comment form arguments.

Where the hook is called

comment_form()
comment_form_defaults
wp-includes/comment-template.php 2286
$args = wp_parse_args( $args, apply_filters( 'comment_form_defaults', $defaults ) );

Where the hook is used (in WP core)

Does not used.