Because the brain stem is what transfers all information to the rest of your body. Also, it is directly connected to the hypothalamus, which if damaged, can take away your ability to breathe or do simple tasks.
The knife, because its a knife
1. Warm up drills
2. Stretching
3. Workout
4. Cool down
D. Stressing defiantly doesn't increase stress. In joints stressing gets released after streching