Quick answer: A fixed floor at 90% of your initial balance that never moves — not up, not down. On $100,000 Pro, the floor is $90,000 permanently.
Grow the account to $150,000 and the floor is still $90,000 — unlike a trailing rule, profit gives you more room, not less. Open losses can still breach it, because it applies to equity.
This is Pro's defining feature next to Speedy's trailing 8%. Flex uses the same static mechanism at 8% — see Maximum Drawdown – Flex (Static).
See also: Daily Drawdown – Pro Accounts