Acharya Prashant explains the practice of negation, or 'Neti Neti', as a tool for spiritual liberation. He emphasizes that negation must occur the moment an internal reaction or identification arises, such as feeling happy, sad, or even claiming to understand a teaching. By saying 'not me' or 'not this', one rejects the ego's constant attempt to assert its existence. He argues that the formula of 'no' is more powerful than 'yes' because humans are burdened by attachments and need to reach a state of emptiness before experiencing true fullness. This negation should not be selective; one must negate both the seemingly positive and negative aspects of life, as the ego holds onto both for its own gratification.