The company has issued shares if i'm not mistaken. Meaning new shares allowed to sell (dilution as well). In times of a hostile takeover attempt they can sell a number of those shares to a 'white knight' ( friendly investor) who won't sell and use their votes in advantage of the company. So the votes will then be over 50% in advantage of the company and the hostile T.O. has no chance of succeeding. If people who have Hostile t.o. intentions know there is a poison pill ( extra shares to bring to market if necessary) they won't even start to try it.