This study comprehensively analyzes and categorizes the etiopathogenesis and pathophysiological mechanisms behind the Kernohan-Woltman notch phenomenon (KWNP), the major cause of ipsilateral hemiparesis (IH), a false clinical localization sign that remains a diagnostic challenge in the era of modern imaging (CT/MRI). For this purpose, a systematic, multilanguage search of articles containing the keywords “ipsilateral hemiparesis”, “Kernohan's notch” and “Kernohan-Woltman notch phenomenon” was carried out in the Pubmed database, in quest of well-described IH/KWNP cases studied with modern radiologic and neurophysiologic techniques (time span from 1977 to 2023). A thorough review of the reference lists from all consulted articles yielded a final cohort of 121 cases for analysis, including two own original patients. The triggering intracranial lesions, mechanisms of ipsilateral hemiparesis, types of motor deficits, midbrain injury patterns and neurological outcomes were all categorized and integrated into a novel etiopathogenic classification scheme, which includes six main IH/KWNP etiopathogenic models grouped under the acronym “CHESTnUt”: “C” for contusive brain peduncle injury (5%); “H” for acute intracranial hematoma, either of traumatic (34.7%) or spontaneous (21.5%) origin; “E” or Ectors model for fronto-temporal masses, mainly extra-axial tumors (8.3%); “S” for chronic or subacute subdural hematoma (19%); “Tn” for tentorial notch/infratentorial mass lesions (6.6%), and “Ut” for uncrossed corticospinal tracts (2.5%), the main alternative mechanism to KWNP for IH proved in the current cohort. Subsequently, multivariate and logistic regression models were employed to identify the clinical-neuroradiological characteristics associated with each model integrating the scheme and the critical factors associated with a favorable motor outcome. Improvement or full recovery from IH after treatment occurred in 70% of KWNP patients, and this was significantly associated with the E, S and Tn models ( P = 0.009). A favorable outcome was also associated with unilateral motor deficits ( P = 0.004), and with absence of a structural lesion involving the contralateral cerebral peduncle (SLCP) on MRI ( P = 0.014). Hierarchical logistic regression analysis selected the combination of three variables: unilaterality/bilaterality of motor symptoms, patient age and SLCP pattern, allowing accurate prediction of IH outcome in 74% of patients. Consequently, the CHESTnUt scheme represents a novel, helpful clinical tool for IH/KWNP classification which allows for a more accurate discrimination of motor outcome in patients presenting paradoxical ipsilateral hemiparesis.