What does it mean that breathing as the fastest regulation tool?

Controlled breathing directly affects your nervous system in seconds.

Example: You are about to walk into a performance review where you will be delivering difficult feedback to a direct report. Your heart rate has climbed. Your thoughts are cycling through worst-case scenarios — tears, anger, an HR complaint. You can feel the activation in your chest and the tightness in your jaw. You are above your window of tolerance, and you know it. If you walk in at this arousal level, you will speak too fast, miss nonverbal cues, and default to either harshness or avoidance. So you stop outside the conference room door. You breathe in through your nose for four seconds. You breathe out through your mouth for eight seconds. You repeat this six times. It takes ninety seconds. By the fourth breath, your heart rate has dropped noticeably. By the sixth, the jaw tension has released and your thoughts have slowed from a cascade to a sequence. You are back inside your window. You open the door and deliver the feedback clearly, calmly, and with the attentional bandwidth to actually listen to the response. Nothing about the situation changed. Your nervous system state changed. And that changed everything about how you handled it.

Try this: Practice the 4-8 breathing pattern right now: inhale through your nose for four seconds, exhale through your mouth for eight seconds. Repeat for six full breath cycles. Pay attention to three things as you do this — your heart rate, the tension in your shoulders and jaw, and the speed of your thoughts. Notice what shifts by the fourth or fifth breath. Then, after a brief pause, try the reverse: inhale for eight seconds, exhale for four seconds, six breath cycles. Notice the activating effect — the slight increase in alertness, the subtle acceleration of mental processing. You just experienced both down-regulation and up-regulation through breathing alone. You now have direct evidence that you can shift your nervous system state in either direction using nothing but the ratio of your inhale to your exhale.