WordPress at a glance
function is not described

_show_post_preview() WP 2.7.0

Filters the latest content for preview from the post autosave.

No Hooks.

Return

Nothing (null).

Usage

_show_post_preview();

Code of show post preview: wp-includes/revision.php VER 5.0.3

<?php
function _show_post_preview() {
	if ( isset($_GET['preview_id']) && isset($_GET['preview_nonce']) ) {
		$id = (int) $_GET['preview_id'];

		if ( false === wp_verify_nonce( $_GET['preview_nonce'], 'post_preview_' . $id ) )
			wp_die( __('Sorry, you are not allowed to preview drafts.') );

		add_filter('the_preview', '_set_preview');
	}
}