WordPress at a glance
filter is not described

the_preview filter-hook . WP 2.7.0

Filters the single post for preview mode.

Usage

add_filter( 'the_preview', 'filter_function_name_4614', 10, 2 );
function filter_function_name_4614( $post_preview, $this ){
	// filter...

	return $post_preview;
}
$post_preview(WP_Post)
The Post object.
$this(WP_Query)
The WP_Query instance (passed by reference).

Where the hook is called

WP_Query::get_posts()
the_preview
wp-includes/class-wp-query.php 2922
$this->posts[0] = get_post( apply_filters_ref_array( 'the_preview', array( $this->posts[0], &$this ) ) );

Where the hook is used (in WP core)

wp-includes/revision.php 552
add_filter('the_preview', '_set_preview');