What does it mean that saying no to protect capacity?

Declining new commitments when at capacity is not selfish — it is responsible.

Example: A product manager receives a request from a VP to lead a cross-functional initiative starting next week. She wants to say yes — it is high-visibility work and the VP rarely asks directly. Instead, she opens her commitment tracker from L-0965 and walks the VP through it: 'My current C/C ratio is 0.92. Adding this initiative at an estimated 10 hours per week pushes me to 1.18. That means something breaks. I can take this on starting March 15th when the platform migration ships, which drops my ratio to 0.74. Or I can start immediately if we reassign the Q2 reporting workstream to someone else.' The VP pauses, then says: 'March 15th works. I appreciate you being honest about it.' She protected her capacity, preserved the quality of her existing commitments, and earned more trust than she would have by saying yes and delivering late on three things simultaneously.

Try this: Identify the next request you receive — professional or personal — that would push your C/C ratio above 0.85 (or further above 1.0 if you are already overcommitted). Before responding, write out three things: (1) your current ratio, (2) what the ratio becomes if you accept, and (3) a capacity-based counter-offer that includes either a later start date, reduced scope, or an explicit trade-off ('I can do X if I stop doing Y'). Deliver the counter-offer using the format: 'My capacity is committed through [date]. I can take this on starting [date], or I can start sooner if we [specific trade-off].' After the conversation, write down the other person's reaction. In nearly every case, the reaction will be more positive than you feared.