Abstract: A method and apparatus is described for modifying downhole wellbore pressure at the instant that an explosive event occurs. By a significant temporary reduction in wellbore pressure, for example, an explosive cutter will be able to cut through thicker pipe in a deep well with high hydrostatic pressure. A favorable transient wellbore pressure is achieved by the selective timing of implosion and explosion devices, to improve the performance of certain explosive tools when subsequently initiated, such as shaped charge explosive cutters, severing tools, setting tools and perforating guns. As part of the system, each implosion or explosion device is initiated by a detonator having a preprogrammed control unit with a preselected timing delay to take advantage of the temporarily lowered wellbore pressure.