I main valor HBG in XX and I posted this before but will repost it again here for you:
To activate valor mode, fill up the valor gauge by firing at the monster. You can fill your gauge faster by using valor reloading.
Once you're in valor mode, you will be granted the ability to do crouching fire and the ability to run and slide after a roll/reload. During a slide or a roll, you are also able to cancel and instantly go into crouching fire mode. Do note that you will lose your valor mode over time and in order to maintain the gauge, you would have to keep firing at the monster.
Below is an extremely high level gameplay demonstrating brave style heavy bow in MHXX for Switch against a max level Bloodlust Diablos. I will spoiler it just in case: