In its default state, the triangle points to the right and the control's contained data is 0. Once the user clicks it, the triangle points down, the control's contained data is 1, and the window that contains the control gets enlarged downwards, by a given amplitude. By default, this amplitude is 0, you can change it using the contextual menu.

The property which stores the amplitude of the enlargement is the control's «class MAXC» property.