Definition of "hydrodynamic"

On the descent from Semmering to Payerbach-Reichenau, with a ruling gradient of 1 in 39 [...], the Voith hydrodynamic brake kept the train within the 19 and 23 m.p.h. limitations without recourse to air brakes, [...].

1961 October, David W. Beadle, “Krauss-Maffei 4,000 h.p. diesel-hydraulic unit for the U.S.A. on trial”, in Trains Illustrated, page 619