When you are mounted, you ride the monster and can. In MHW, Hunters can jump and grab hold onto the monster, effectively 'mounting' it. You can do a regular wirebug vault to send your hunter into the air and hit a monster with your weapon. Once mounted, you can attack it for as long as you stay held on. Hitting a large monster with a jumping attack gives you a chance to mount it. It is best to reserve this for when the game instructs you. When mounting the monster you can attack the monster. If successful, you will initiate a mount. Attack the monster's body, head, or tail while airborne (weapon dependent, but Circle / B, Triangle / Y or the right trigger). Find a way to launch yourself into the air (by running off a high ledge, for instance).
