Sentence:  They're called "<WSD>power walls</WSD>"—those highly visible shelves behind convenience store cashiers. Tobacco companies pay big bucks to use them ($100 million in Canada in 2005), admitting the walls are their single most effective advertising tool. 
Meaning of the word marked with <WSD> tags:  Behind retail sales counters in jurisdictions where tobacco advertising has been banned or heavily restricted, a prominent, enticing display of tobacco products.