Reverse Charge Mechanism triggers promoter liability when input procurements from registered suppliers fall below threshold, requiring cash payment. Reverse charge applies where a promoter of specified real estate projects has not opted for alternative notified tax rates and procures less than 80% of inputs and input services from registered suppliers during the financial year; tax on the shortfall and on cement or capital goods from unregistered persons is payable under RCM in cash, computed at year-end and remitted electronically using DRC-03 within the prescribed period, with project-wise records maintained and unavailed input tax credit reported as ineligible.