Code II (A) - Nonpublic information

“prohibition of all proprietary trading while a firm is in possession of material nompublic information may be inappropriate because it may send a signal to the market. In these cases, firms should take the contra side of only unsolicited customer trades.” I can not image how is “signal to the market”? how firms take the contra side? and what is unsolicited customer trades? Can anybody put forth examples to illustrate the above? Many thanks!