Lets say you purchased 10 BTC at 40 bucks. Losing 400 won't break the bank. Now it crossed 120. If you sold 4 BTC you'd still have 6 BTC and make 80 bucks in the process. The $80 isn't as important as the fact that you've already effectively reduced the cost basis of the 6 BTC to zero, in which case you literally could just sit there and wait. If the price did crash to zero, you would still be net ahead.