A premature clone is a cloned object that has some of its properties not fully loaded yet. This occurs on the first frame a nearby spawner spawns objects that can be cloned, and on that frame, the object would temporarily be placed at the origin of the course (0, 0, 0) while many of the properties are missing.
A clone of such an object mimics many of the same traits. Therefore, a premature clone would, if released, be placed at the origin of the course while missing some effects. For example, a coin released prematurely will not be collected while the object is released, unlike the standard steps of cloning a coin. Using this technique, Pannenkoek2012 could complete some stages without collecting any coins where it may seem inevitable, notably in Mario Wings to the Sky.