The difference between quantum mechanics and classical physics that allows for a nonlocal effect in the former but not the latter is primarily related to the concept of entanglement.
In classical physics, objects are assumed to have definite properties and behave independently of each other. The principles of classical physics, such as Newtonian mechanics, are based on local interactions. This means that the behavior of an object is determined by its immediate surroundings, and any influence on one object does not instantaneously affect another object at a distance.
In contrast, quantum mechanics introduces the phenomenon of entanglement, which allows for nonlocal effects. Entanglement is a property where two or more quantum systems become correlated in such a way that the state of one system cannot be described independently of the state of the other, regardless of the physical distance between them. When two particles are entangled, measuring one particle's state instantaneously affects the state of the other particle, regardless of the distance separating them. This instantaneous correlation between entangled particles is known as "quantum nonlocality" or "spooky action at a distance."
This nonlocal behavior is one of the fundamental aspects of quantum mechanics that sets it apart from classical physics. It has been experimentally observed and verified in various quantum systems, such as the famous Bell's theorem experiments. However, the exact mechanism behind this nonlocality and the nature of the instantaneous correlations are still subjects of ongoing research and debate in the field of quantum physics.