One of the slightly hidden mechanics in Ghost of Tsushima is that you can observe enemy leaders and captains before you engage in combat with them. This allows you to earn extra progress toward unlocking your stances, powerful techniques that allow you to counter different types of enemies and weapons.
How to observe enemies
You can only observe enemies while they are unaware of your presence, so you need to be very careful not to get spotted. You will need to very sneakily make your way through an encampment, and get within 25 meters of an enemy leader if you wish to observe them.
Finding the enemy leader is much easier if you take advantage of your sharpened senses. Press on the touchpad to enter the sharpened sense mode, and you will be able to see red outlines of enemies that are close to you. The enemy leaders will normally be swinging their weapons as if they are practicing fighting, and will be wearing thick armor compared to the rest of the troops in the camp.
When you get close to the captain, you will be prompted to hold the R2 button to observe them, and you will need to do this for five seconds, unbroken. Ensure that no patrolling enemies are heading for your hiding spot before you do this.
Remember, you can use tall grass to hide your presence, sneak under houses, across rooftops, and even wear specific armors that will give you bonuses towards your stealth to make this easier. It is worth doing as often as you, as it will double the speed at which you get new stances.
Every time you observe an enemy, you will gain one stage towards the next stance, then killing that enemy will provide another stage. If you just kill the captains and leaders before observing them, you will only be making half the progress towards your next stance that you could be making.