A dividing cell must make transitions that are decisive and irreversible: once it commits to divide, it cannot drift back. The engine that does this is cyclin bound to a cyclin-dependent kinase — CDK — and the trick is positive feedback.
Active CDK switches on its own activator (Cdc25) and switches off its inhibitor (Wee1), so active CDK makes more active CDK. That self-amplification turns a smooth rise in cyclin into a switch: activity holds low, then snaps high at a threshold.
Feedback also gives the switch a memory. It turns ON at a high cyclin level but stays ON far below that — flipping OFF only at a much lower level. The gap between the two is hysteresis, and it is why a checkpoint is a commitment, not a dial.
The real cell forces the OFF-jump by destroying cyclin B — abruptly, once per cycle — so it can never slip back across the door it just walked through.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.