The paper presents a parametric experimental study of explosion initiated in a vessel and vented through a duct. The aim is to clarify the mechanism of the vessel-duct mutual interaction during explosion and its role in determining the overpressure in the vessel. For a vessel of fixed size, the chosen parameters are the diameter and length of the discharge duct and the profile of area change between the vessel and the duct. Results with a stoichiometric propane-air mixture demonstrate that the change of the combustion regime inside the vessel, responsible for augmented overpressure, is driven by an impulse generated in the initial part of the duct (during an explosion-like combustion) shortly after the flame penetrates into it. The origin of the impulse is discussed. Increasing the duct/vessel diameter ratio or using the profiled vessel-duct passage weakens the generated impulse (decreases its pressure and velocity amplitudes) but at the same time generates higher subsequent pressure rise in the vessel. This implies that enhancement of the explosion overpressure in the vessel is unavoidable in this type of venting.