In removable appliances, the force applied on the upper incisor crown creates a moment that tips the tooth around a center of rotation located approximately at the middle third of the root. This is because the point of force application (usually at the crown) is away from the center of resistance, causing the tooth to rotate rather than translate bodily. The center of resistance of a single-rooted tooth is generally found near the middle third of the root, hence the tipping occurs around this point. Applying force at the incisal edge or apex would result in different movement mechanics, but clinical studies and biomechanical models confirm that in removable appliance tipping, the center of rotation is near the mid-root area.

Reference: Proffit, Contemporary Orthodontics, 6th Edition.