From the NYTimes:

Every soldier on election duty heard intelligence warnings that insurgents would try to slip bomb-laden suicide vests into polling places beneath the long gowns of an Iraqi woman or of a man in woman’s clothing. That presented a particular difficulty in a society where it is not acceptable for a man to search a woman, and there were hardly enough women in the Iraqi Interior Ministry to spend a day at every polling site conducting body searches.
But American officers devised a solution. They agreed on a plan with Iraqi security forces, who were the visible presence inside each polling place, that one of the first women to arrive at larger polling places would be searched, and that woman would in turn be asked to search 10 others. One of those 10 would then search 10 others before voting, and so on in a daisy chain.