Hemoglobin binds four oxygen molecules, and each one bound makes the next bind more easily — cooperativity. That is what bends a plain saturation curve into a sigmoid, and it is one of the most consequential graphs in physiology.
The flat top is a safety margin: in the lung, even a large fall in alveolar PO₂ barely dents saturation, so loading stays near-complete. The steep middle is a delivery engine: in tissue, a small drop in PO₂ unloads a large slice of oxygen right where it is consumed.
Working muscle is acidic, warm and rich in CO₂. Each of these slides the whole curve to the right — the Bohr effect — so at the same tissue PO₂ hemoglobin lets go of more oxygen. Delivery is tied to demand. Strip the cooperativity away (n → 1) and the curve slumps into a lazy hyperbola that can neither load fully nor unload generously.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.