Arbitration is by definition a low-risk strategy was entrusted to him by Leeson and his team in order to produce small profits, earnings and not to speculate.
Leeson's experience in stock transactions was very low but gave an exam that qualified to speculate on SIMEX along with his colleagues.
Senior bank managers came mainly from a commercial bank and they had neither enough knowledge about trading. Even in the face of high profits, it would have sounded the alarm over the high risks they involved, managers continued to believe that Leeson has covered positions on SIMEX and OEX, thus making a profit un-risky.