As a start read this from the FT
Two banks, JPMorgan and
Deutsche Bank, account for about 20 per cent of total global derivatives exposure. Each has more than $50tn potentially at risk. The current market capitalisation of JP Morgan is about $200bn (roughly its book value); that of Deutsche, $23bn (about one third of book value). From one perspective, Deutsche Bank is leveraged 2,000 times. Imagine promising to buy a house for $2,000 with assets of $1.
Before you head for the hills, or the bunker, understand that there is no possibility that these banks could actually lose $50tn. The risks associated with these exposures are largely netted out — that is, they offset each other. As you promised to buy the house in question, you also promised to sell it: though not necessarily at just the same time or price or to the same person. That mismatch is the source of potential profit.