By David Russell, reporter at OptionMonsterNEW YORK -- Chicago Bridge & Iron ( CBI) has been pulling back hard, but Tuesday the buyers stepped in. OptionMonster's trade scanners detected unusual activity in the May 55 calls, with large blocks purchased for $1.40. More than 5,600 contracts changed hands, clearly new positions because open interest in the strike was just 734 before the session began. These calls lock in the price where shares can be bought in the engineering company. Their cheap price can give them significant leverage, letting investors position for a rally at low cost. The stock was down more than 1% when the calls hit but then rebounded and closed up 0.17% to $52.64. The calls appreciated some 10% to $1.55 from that small move. Earnings come out May 2 and the last report beat on the top and bottom lines. The stock is down more than 15% since late March, so Tuesday's traders were apparently looking for a bounce heading into the next release. Total option volume was twice the daily average in the session, with calls outnumbering puts by more than 5 to 1. Russell has no positions in CBI.