My understanding is that instead of having employees take a secret ballot, it allows union organizers to go around “asking” employees if they want to organize. The employees under duress are then “asked” to fill out signature cards. When the union thugs get signature cards for over 50% of the employees they are allowed to “organize” the rest.
Of course, this type of abuse can go the other way too. Politically connected companies can squeeze out the unions as long as they got pull. Either way, workers lose.