Sekiro: 18 Tips & Tricks The Game Doesn't Tell You
11. This Green Icon During Combat Triggers A Unique Animation
Sekiro half-indicates this mechanic during base gameplay, in that the grapple hook icons that adorn the environment change from grey to green when they can be interacted with using L2/LT.
However, that's also the same for enemies. Mini-bosses like the ogre or horse-riding Gyoubu Oniwa have windows of opportunity where the grapple icon will appear above their heads, letting you know there's a unique animation available.
In the ogre's case it's a head-kick that can staggers him and allows for a followup combo, whereas in Oniwa's fight, the grapple is only used to close distance, getting you up-close for a quick attack before getting out his way again.
Mastering Sekiro's combat means mastering all combat options at your disposal, so don't ever miss these button prompts.