Your teeth are anchored in your mouth with bone, tissue, and roots. To move your teeth, you have to move your teeth through these structures. Bone cells and other tissues are stimulated via pressure of the trays which causes them to be broken down and pushed out of the way to allow the tooth to move. The tooth then needs to be held there while the bone cells and supporting tissues fill in around the tooth's new position and harden. This takes time, and should not be done too quickly.
This is why you generally need to follow the 20-22 hours a day, 2 weeks per tray schedule. If, for example, you did less hours per day, or switched out the trays after a week, your results will be very unpredictable. Even if it seems or feel like it would be ok, it might not be. If you do not follow the prescribed schedule, some teeth may move, others not, and the results would likely not reflect the original plan, and it can adversely affect for your long-term dental health.