A method for operating at least one pump in a pump assembly, comprising: providing a pump assembly comprising a fluid end and a power end, the fluid end in communication with a fluid source and at least one downstream destination and comprising a pump housing for a pressurizable chamber, at least one valve (250, 255) comprising a suction valve and a discharge valve for controlling fluid communication with the pressurizable chamber, the at least one valve (250, 255) defining a normal duration of allowing fluid communication with the pressurizable chamber during operation of the pump assembly; providing a valve actuation guide (100, 200, 300, 450) external to the chamber, the valve actuation guide (100, 200, 300, 450) coupled to the at least one valve (250, 255) and operable to assist in controlling fluid communication with the pressurizable chamber; operating the pump assembly; and actuating the valve actuation guide (100, 200, 300, 450) to change an aspect of the valve duration, wherein the fluid source comprises a fracturing fluid, wherein the pump assembly is a multiplex positive displacement pump assembly comprising a plurality of plungers and cylinders (610A, 610B and 610C); characterized in that: the valve actuation guide (100, 200, 300, 450) of each of the cylinders (610A, 610B, 610C) is actuated to vary the valve duration of the suction valve and the discharge valve in order to generate pulses with the fracturing fluid within a wellbore to improve or enhance fracture propagation within a formation, wherein the opening of the at least one discharge valve and/or the at least one suction valve is synchronized to generate the pulses within the fracturing fluid. providing a pump assembly comprising a fluid end and a power end, the fluid end in communication with a fluid source and at least one downstream destination and comprising a pump housing for a pressurizable chamber, at least one valve (250, 255) comprising a suction valve and a discharge valve for controlling fluid communication with the pressurizable chamber, the at least one valve (250, 255) defining a normal duration of allowing fluid communication with the pressurizable chamber during operation of the pump assembly; providing a valve actuation guide (100, 200, 300, 450) external to the chamber, the valve actuation guide (100, 200, 300, 450) coupled to the at least one valve (250, 255) and operable to assist in controlling fluid communication with the pressurizable chamber; operating the pump assembly; and actuating the valve actuation guide (100, 200, 300, 450) to change an aspect of the valve duration, wherein the fluid source comprises a fracturing fluid, wherein the pump assembly is a multiplex positive displacement pump assembly comprising a plurality of plungers and cylinders (610A, 610B and 610C); characterized in that: the valve actuation guide (100, 200, 300, 450) of each of the cylinders (610A, 610B, 610C) is actuated to vary the valve duration of the suction valve and the discharge valve in order to generate pulses with the fracturing fluid within a wellbore to improve or enhance fracture propagation within a formation, wherein the opening of the at least one discharge valve and/or the at least one suction valve is synchronized to generate the pulses within the fracturing fluid.