Consistency rule (typical form)
No single day can represent more than X% of your total profit (often 30–40%). This forces you to earn the account across multiple days, not gamble it in one.
Scaling plan
Some firms require you to trade smaller until you cross specific profit milestones. Ignoring the scaling plan can void your funded account.
The lesson
Prop firm rules are designed to reward consistency over heroics. Small, boring, repeated green days beat one giant winner every time.