A pinkie swear is a way of sealing a promise and functions as a form of natural Thinkamancy. They are binding and compel a unit to keep their promise in a manner similar to an order. Breaking one requires a significant amount of willpower, and can generally only be done if their life is at stake, which will result in an icy cold headache. They require a small amount of juice to make, and as such, prisoners cannot make them.
Real World References
See the Wikipedia pinky swear article.