Additional tax on understated income mandates an extra levy, offsets refunds, and adjusts demand or refund following revisions. Amendment caps intimation issuance under section 143 at two years from the end of the assessment year when income was first assessable. It introduces sub-section (1A) requiring the Assessing Officer to levy an additional income-tax on the tax attributable to any excess income determined by adjustments under the proviso to clause (a) of sub-section (1), to offset refunds by that amount, and to increase or reduce the additional tax following specified revision or rectification orders, with demand or refund consequences. The Explanation prescribes how the tax on the excess amount is to be measured in alternate scenarios.