A 72-year-old woman from Bangalore with atrial fibrillation (not on anticoagulation) presents with acute onset right-sided hemiparesis and aphasia at 6 hours after symptom onset. Non-contrast CT head is normal. MRI DWI shows multiple acute infarcts in the left MCA and ACA territories. Perfusion-weighted imaging (PWI) shows a larger area of hypoperfusion than the DWI lesion. What does the PWI-DWI mismatch represent, and what is its clinical significance?