WordPress at a glance
action is not described

update_postmeta action-hook . WP 2.9.0

Fires immediately before updating a post's metadata.

Usage

add_action( 'update_postmeta', 'action_function_name_7579', 10, 4 );
function action_function_name_7579( $meta_id, $object_id, $meta_key, $meta_value ){
	// action...
}
$meta_id(int)
ID of metadata entry to update.
$object_id(int)
Object ID.
$meta_key(string)
Meta key.
$meta_value(mixed)
Meta value.

Where the hook is called

update_metadata()
update_metadata_by_mid()
wp-includes/meta.php 247
do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
wp-includes/meta.php 705
do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );

Where the hook is used (in WP core)

Does not used.