Visual closure is the ability to perceive the whole image when only part of the image is available. Visual closure builds several important skills later in life -- math, and reading, among others. The Play Kit builds on this ability when the pegs in the Play Kit seem to 'disappear' in the forms, the child realizes that the centre hole in the forms disappears too and the shape becomes a whole.
Book: Pooni, Pooni, Where Are You? Play Kit: Channapatna Forms on Pegs