A turbocharger system includes first and second turbines arranged such that exhaust gas passes through the first turbine then the second turbine. A bypass channel is configured such that exhaust gas entering the channel passes only through the second turbine. A valve positioned in the bypass channel regulates the flow of gas therethrough. The valve accelerates a stream of gas and focuses the stream toward the second turbine such that a large part of the added velocity of the stream is preserved as it enters the second turbine. Operation of the valve may be controlled so as to maintain the valve in a closed position while exhaust gas pressure above the first turbine pressure is below a first threshold, to progressively open the valve as the pressure increases above the first threshold, and to maintain the valve in a full-open position while the pressure is above a second threshold.