"Free association" in psychoanalysis just asks people to let themselves speak, to fall into their speech, let it unfold - but it is very, very hard to do, because what you say tends to not be what you want to say. You want to avoid certain things, you can see your chain of speech leading up ahead to somewhere you don't want to go. So you stop yourself, or resist, and then what you are saying is not at all free but strained, hedging, tense and anxious.