Basically, everyone knows what society expects of them. When you respond quickly, you default to doing what other people expect of you. When you have time to think, then you can choose to respond in a way that fits with who you are.
The reason this pattern is interesting is that we often have to react quickly in social situations. To help us get along with other people, those quick reactions are focused on saying and doing the things that are likely to make it easier for us to be liked by other people. Only when we have time to react slowly do we like to buck the trend of what other people expect.
This work suggests that if you are in a social group that is doing something you would like to avoid, then you need to find a way to slow your actions down to say and do things that are desirable for you—even if they go against the grain of the rest of society.