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

Null. Nothing.

Usage

_show_post_preview();

Changelog

Since 2.7.0 Introduced.

Code of show post preview: wp-includes/revision.php WP 5.2.2

<?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.' ), 403 );
		}

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