People often look at their own behaviour to decide what their beliefs are. In getting someone to agree to your small initial requests, you’ve helped shape their self perception to include the belief that “I am the sort of person who…..“. They are then much more likely to comply with other request. Asking a person to wear a small pin about breast cancer awareness does little on and off itself, but the person who agreed to wear it is now more likely to believe “I am the sort of person who cares about breast cancer” That belief makes it easier for breast cancer groups to later solicit that person to volunteer time or donate money to combat breast cancer.

The foot in the door effect is hotly debated in the world of advocacy. With some activist arguing that encouraging small changes is bad because it makes the public complacent and dissuades them from more meaningful changes. Others argue that not only do small changes produce immediate good, but they make it more likely that people are willing to later on engage in larger changes. So, which side is correct? First the foot in the door only works if the second request is actually been made. We shouldn’t assume that people then go on to take further steps on their own.
Second, there is a threshold over which this effect doesn’t work anymore.

Read more about how this tactic works: