You can increase Jin's resolve by slicing bamboos in Ghost of Tsushima. You have to enter a combination of 3 hits, then 5 and finally 7 very quickly.
There are 16 training bamboos to find in total.
Tip: If for some reason you want to make things easier for yourself, you can go to the "Options" of the game, then to the "Accessibility" menu and activate the "Simplified Commands" line. This way, there will no longer be a time limit for entering the button combination.
Unlocking the Bamboo Strikes
You'll have to progress through the story or free a village in order for the nearby training bamboos to appear. This is the case for the bamboos of Azamo and Komatsu, for example. As you will see, the majority of bamboos are close to a city, but there are a few exceptions.