Felix Salmon offers the best explanation I have seen:
When basis trades blow up: The basis trade is an arbitrage…. There are two different ways the market measures credit risk: by looking at credit spreads — the yield on a certain issuer’s bonds, relative to the risk-free rate — or by looking at CDS spreads, which are basically the same thing but set in the derivatives market rather than the cash bond market. Most of the time, CDS spreads and cash spreads are tightly coupled. But sometimes they’re not….
This time around, the basis-trade disaster has happened at JP Morgan, where the famous London Whale seems to have contrived to lose $2 billion on what was meant to be a hedging operation. And once again, although the details are still very murky, the culprit seems to be the CDS-cash basis….
Bruno Iksil, the London Whale, had a massive long position on corporate CDS in general, and the CDX.NA.IG.9 index in particular. He was selling protection, betting that credit spreads would go down, rather than up. The position was meant to be a hedge, although it’s a bit unclear how JP Morgan could have some massive short position in corporate debt that it was hedging against. In any case, CDS spreads went up — and credit spreads, in the cash market, didn’t.
Cue a $2 billion loss.
Rarely has a position been as widely publicized as Iksil’s, and I wouldn’t be at all surprised to learn that the credits with the highest basis were precisely the credits CDX.NA.IG.9 index. Whenever a trader has a large and known position, the market is almost certain to move violently against that trader — and that seems to be exactly what happened here…