It works the same way as inoculation delivered in a syringe.
You give your audience a small dose of the opposition's argument to stimulate their defences.
Let's say you want to check whether members of the social club think it would be a good idea to hold a big party next month. You put out a questionnaire (or use Survey Monkey as we do). Most people reply yes, a party sounds great to them.
How many will come?