Cool - Keep Your Cool

When facing a stressful situation, even a highly skilled character can lose their cool. Scenes such as navigating a rocky coast, going through a burning building, or acting before a timed explosive detonates are all situations in which stress can make a character lose their cool and perform worse than what are expected of them.
The GM should call for a Cool check when a character is facing a stressful situation. A character should only roll once for the entire scene unless the situation gets even more stressful.
Many situations are stressful yet do not require checks. After all, every time you make a check, your character has a possibility of failure and to some extent, that could be stressing them out. Yet, you do not want to slow down the game each time a character has to make a check so they would have to make a Cool check prior to it. Combat situations, unless there are some specific circumstances, should not require a Cool check even though being engaged in a shoot-out is a stressful situation. Same goes for making that jump over a chasm or sneaking past the guards. The GM should only call for a Cool check when additional stressors are present. For example, trying to pick a lock to open a door would not require a Cool check. But if your character is stuck in a room rapidly filling with water and the only way out is unlocking that door, the GM should call for a Cool check before your attempt.
Depending on the situation, some characters might not need to roll or have the difficulty decreased as per their background and personality. For example, a trained sapper should not face the same difficulty for disarming a bomb than an untrained person trying to recall instructions they received earlier that day on how to disarm that same bomb. 
Cool vs. Fear 
Succumbing to stress is similar to succumbing to fear: the character will lose their nerve. They might even let their base instincts dictate a specific course of action.
Even though both fear and stress can inflict debilitating penalties, they have a different trigger. When a character is facing a stressful situation in which they are still in control, it calls for a Cool check, even if the situation is life-threatening.
Disabling an explosive is a great example as the character is still in charge and it is up to them to manipulate the device the right way to disarm it.
On the other hand, finding yourself surrounded by sharks while swimming calls for a fear check. The situation is life threatening and the character has no control over the sharks' presence.
Example of Keep Your Cool
Heidi and Elysha are soldiers fighting in an ongoing war. Since everything started, they always looked to each other and saved each other's lives multiple times. Today they are being sent to a ruined portion of a conquered city to fix arcane cabling so the arcane nodes can be reestablished.
The two cautiously dash from cover to cover even though there is no sign of immediate danger. Heidi is leading while Elysha runs behind her.
At some point, as they were moving between cover, they hear a gunshot. Heidi's instinct has her jump behind the cover. She turns to look at Elysha right behind him but is faced with a grim sight. The alvain is lying on the floor, her eyes closed and Heidi can see blood flooding out from the woman's neck.
The GM calls for a Cool check. The reasons behind is that the apparnet death of her companion adds an element of stress to an already tense situation. Heidi is suddenly alone and pinned down. Yet, she is still in full control. She could theoretically take time to cope with what just happen while standing safe behind cover, and then engage the enemy. This justifies calling for a Cool check rather than a fear check.
Elysha being one of Heidi's close friends, the GM sets the difficulty at Hard (ddd). Heidi has the Failure motivation. Both women have taken an unspoken oath between themselves to look out for each other and never let the other down. Even though she did not pull the trigger, Heidi feels like she failed her friend. Therefore, the GM upgrades the check once.
The roll generates faad. Since the roll failed, the GM says that Heidi is in shock and will simply stay behind cover, staggered. Heidi's player suggests that she could spend a to have her character spot the muzzle flash when the gunshot occurred and thus could pinpoint the shooter. The GM agrees and then suggests that instead of cowering behind cover, Heidi might be struck by a sudden compulsion to avenge her fallen comrade, and will counterattack even though distance is not on her side. Heidi's player spends the remaining a to have her character have a moment of focus while trying to fight back, adding b to the check. Finally, the GM spends the d to have Heidi move out of cover as she stands up and walks toward the sniper, as her compulsion to avenge her sister-in-arms surpasses her sense of self preservation.