It stops opponent’s cooldown. It is a suggestion because common builds nowadays rely on fast charging specials so they dish out specials at you a lot. With guard, you slow down their charging or stop them altogether as long as you are within the HP threshold(80% above if I remember correctly). For example, I’m running a Nowi with guard. Opponent with a Brave Lyn chooses to attack me. Let’s say B!Lyn is using Draconic Aura for her special. It starts at cooldown 3 if I remember correctly. If I’m running guard, her special won’t charge as long as I’m above HP Threshold. So it will remain at 3 the whole time and she can’t get to hit me with Draconic Aura since it won’t charge. My Nowi’s special will charge unless that B!Lyn is running guard as well. When using skills like Steady Breath to speed up charging, guard will reduce the charge to 1 instead of 2 slowing down the charging.