Borrow in a currency that pays you almost nothing, put the money into something that yields more, keep the difference. When the yen sits near zero and the dollar pays 5%, borrowing yen to hold dollar assets captures that gap. On its own that is a spread. Multiply it across hundreds of billions in institutional money and it becomes one of the quieter forces steering where global capital actually goes.
How the trade works
The mechanics are simple. Borrow 1 billion yen at 0.1%, convert to dollars, buy Treasuries yielding 4.5%. You earn the 4.4% spread minus costs. Now add leverage. Plenty of hedge funds and institutional desks run carry at 5-10x, which turns a modest yield gap into a real return.
The catch is the currency you borrowed in. If the yen strengthens 5% while you are earning 4.4% of carry, you lose money the moment you convert back. That currency risk sits inside every carry trade, quiet until it is not, and when it goes off the moves are violent.
What makes it worse is that the trade feeds itself on the way up. Borrowing and selling yen weakens the yen, which makes the trade more profitable, which pulls in more borrowing and selling. That loop pushes positioning to extremes, and extremes are exactly what set up sharp reversals.
When it unwinds, risk assets crash
Run the loop backwards and you get the crash. A small yen move up nudges a few traders to close, closing means buying yen, buying yen strengthens it further, which forces more closures. The cascade produces sudden drops in risk assets that look unexplained, because the selling has nothing to do with the assets and everything to do with the currency underneath them.
August 2024 was the clean example. The BOJ nudged rates up, the yen strengthened, and the carry unwind sent global equities down hard, with crypto down even more because of its higher beta. None of it was about crypto fundamentals. It was leveraged currency positions mechanically getting torn out.
For crypto traders these unwinds behave like a macro flash crash. Sudden, severe, usually temporary. The move is to not panic-sell into them, and if you are sitting on dry powder, to treat them as buying opportunities once the worst of the unwind is behind you.
Reading the risk before it hits
A few things tell you how loaded the trade has gotten:
- The size of speculative short positions in funding currencies like the yen and Swiss franc, reported in COT data. Extreme yen shorts mean the unwind risk is high.
- The gap between short-term rates in funding currencies and target currencies. Wide gap, strong incentive, trade keeps growing. Narrowing gap, whether the funding country hikes or the target country cuts, and the incentive fades while unwind risk climbs.
- Central bank talk, mostly the BOJ. When it signals normalization through higher rates or less QE, it is aiming straight at yen-funded carry. If you hold real risk exposure, crypto included, BOJ minutes and governor speeches are worth actually reading.
The crypto version of the same trade
Crypto runs its own carry. Borrow stablecoins, which pay relatively low interest in some protocols, and push that into higher-yielding DeFi strategies or leveraged positions. When DeFi yields compress or lending rates spike, that carry unwinds too, and it feeds the deleveraging cascades that make price drops worse.
The part worth watching is where the two overlap. Yen-funded carry and stablecoin-funded carry can unwind at the same time and compound, which is roughly what happened across several 2022 liquidation events. Tracking both gives you a fuller read on how much leverage is actually sitting in the system and how disorderly the exit could get. It is the same instinct behind watching cross-asset positioning on Blockcircle rather than any single market in isolation.
Practical takeaway
Keep the yen-dollar rate and yen short positioning as a background gauge. When yen shorts are stretched and the BOJ is signaling a shift, trim risk exposure ahead of it rather than waiting for the unwind to land on you. Being a little early costs you some upside. Being caught in a carry unwind at full size costs a lot more.