There is both foot-in-the-door phenomenon and foot-in-the-door technique. As you can guess, the technique is used to get the phenomenon. The phenomenon is the tendancy for people to comply with some large request after first agreeing to a small request. As you can then imagine, the technique is used to get compliance from others (to get them to behave in a way you want) in which a small request is made first in order to get compliance for a larger request. For example, someone might want you to give to give 5 hours of your time a week for the three months as a volunteer to a charity (a big request). But to get you to agree to this big request, they first ask you to volunteer for 1 hour one time and one time only. After hearing this small request, which you are willing to agree to, they then work their way up asking you to volunteer time until you are willing to agree to the big request. You are more likely to agree to this when you have already said yes to the small request.