WordPress at a glance
function is not described

_set_preview() WP 2.7.0

Sets up the post object for preview based on the post autosave.

No Hooks.

Return

WP_Post/false.

Usage

_set_preview( $post );
$post(WP_Post) (required)

Code of set preview: wp-includes/revision.php VER 5.1.1

<?php
function _set_preview( $post ) {
	if ( ! is_object( $post ) ) {
		return $post;
	}

	$preview = wp_get_post_autosave( $post->ID );
	if ( ! is_object( $preview ) ) {
		return $post;
	}

	$preview = sanitize_post( $preview );

	$post->post_content = $preview->post_content;
	$post->post_title   = $preview->post_title;
	$post->post_excerpt = $preview->post_excerpt;

	add_filter( 'get_the_terms', '_wp_preview_terms_filter', 10, 3 );
	add_filter( 'get_post_metadata', '_wp_preview_post_thumbnail_filter', 10, 3 );

	return $post;
}