Their theory is that, contrary to Bolshevik-style political commissars, "free" societies like the bourgeois democracies rely on less overt and obvious (but even more insidious) forms of censure to keep unwanted ideas out of the popular consciousness. The end result of this filtering process is to reduce popular dialogue to a wafer-thin "consensus," representing the attitudes of the power elite who run our society. Everything else is considered "beyond the pale" or simply never discussed at all. This method has been in practice for many decades.
Part of your inoculation against this filtering is to be aware of it in the first place. The greatest strength of propaganda is when it isn't recognized as such.
It'll take me some time to get all the information down here, so I'll start with a list, for your reference, and then will add more material to it over the next few weeks.