As an abritrageur, Kerviel was supposed to purchase one Portfolio of stock index futures and simultaneously sell a similar mixture of index futures with a slightly different value , as a hedge. But while Kerviel , according to the bank, put sizable, real purchases in one portfolio , he created fictitious sales transactions in the second, offsetting portfolio. This gave the impression to risk managers that Kerviel was arbitraging between the two portfolios and that the risks in the first portfolio had been hedged when in fact they had not been.