That's because a sustained fall to under 6.20 could cause losses on billions of dollars of hedging products that Chinese companies have taken out as part of a bet on the renminbi's appreciation.
Strategist Geoff Kendrick estimates that mark-to-market losses on the products top $2B.
The offshore yuan, to which the derivatives are tied, is at 6.1905.
The onshore currency is at 6.1965 after falling to as low as 6.2026, with the movement being the first time the renminbi has deviated more than 1% from the central bank's daily midpoint after it doubled the currency's trading band to 2% from that point over the weekend. The bank set today's midpoint at 6.1351.