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.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

No Hooks.

Return

null. Null. Nothing.

Usage

_show_post_preview();

Changelog

Since 2.7.0 Introduced.

Code of _show_post_preview() WP 5.7

<?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' );
	}
}