Pledge is cast under the consent of the magician, it limits the mental activity of a chosen subject semi-permanently. The mind of a target cannot be bound unilaterally, for a "key" exists to release the partner’s independent will. It's a magic which enables partial control over someone while still maintaining the emotional state of the controlled subject. 
Tatsuya is the only known subject for this magic to be used on, and Miyuki being the "key" to releasing it. A side-effect of the magic is less control of her powers and so causes Miyuki's power to run rampant when she becomes emotional.