How did they film the chasing scene (Bronson, Mifune, hill slope)?

I've always wondered how they filmed Toshirô Mifune's character, almost standing straight - i.e. not crawling or climbing down, chasing Bronson in the hill scene where Bronson tried to escape.. I guess that was a stunt double but still, that stunt double did a really great job, or, did they use a rope or something to keep him that straight up?