In FX you are always trading one thing against another. There is a long position, and a short position. In this case, you were long EUR and short CHF. When CHF

In FX you are always trading one thing against another. There is a long position, and a short position. In this case, you were long EUR and short CHF. When CHF revalued 15%, you had to pay many more euros to buy back your short. So if you started with 100 EUR long, 120 CHF short, at the pre-break exchange rate of 1.2, then when it "cracked" to say 1.05, you had to pay 120/1.05 to repay your short. That's 114.2857 EUR you had to pay back, but you only had 100 EUR to start out. Thus you lost 14+%. By the way, poorly explained in the main post: most of the stops actually happened way below 1.05. The cross panic-traded down to 0.85 in the hour after the reval announcement, which is where most of the stops were occurring. 1.2/0.85 - 1 = a 41% loss. Imagine if you were leveraged 100x......