Japan's decades of near-zero interest rates quietly bankrolled trillions in investments worldwide, and that setup has been good for anyone borrowing cheap yen.

• For 20 years the rest of the world printed money fast, but Japan barely grew its money supply and held interest rates at zero.
• That let banks, hedge funds, and investors borrow yen for free and swap it into dollars.
• They used that borrowed money to buy US government bonds paying 4-5%, tech stocks, and Bitcoin — basically free profit.
• This "yen carry trade" is estimated to have funded trillions of dollars of bets across global markets.

Outlook: This cheap-money engine only works while Japan keeps rates low, so any move by Japan to raise them could force investors to unwind those bets and shake markets worldwide.