The mechanism controlling occurrence of the low-velocity denotation in obstacle-laden systems is discussed. A quasi-one-dimensional model where the impact of obstacles is accounted for through a drag-force term is explored. It is shown that the hydraulic resistance gives rise to a powerful agency (diffusion of pressure) capable of driving the combustion wave both at fast subsonic and at supersonic velocities. The latter mode is identified with the so-called choking regime often observed in obstacle-affected detonations. (C) 2003 Published by Elsevier Inc. on behalf of The Combustion Institute.