User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-21371438-20130922214631/@comment-11537809-20130922215332

You have to jump from a ledge higher from the monster and successfully land a hit on it (doesn't always let you mount the monster, you can tell if it falls down that it was successfull). Lance and Insect Staff users can both jump on a monster's back without being on a higher platform (Lancers can jump during their charge attack).