It has been disclosed as a cross trade.E.G A broker has a client who wishes to sell 1 share at $10,another wants to buy at $10,the broker simply puts through a cross trade.Another example,a client went short at $10,it drops to $9,he wants to take profits,yet wants to remain short,so a cross at $9 closes his short at $10,and reopens a short at $9,result $1 profit and short again at $9.