The stopped-flow method is a development of the continuous-flow method used by Hamilton Hartridge and Francis Roughton to study the binding of O to hemoglobin. In the absence of any stopping system the reaction mixture passed to a long tube past an observation system (consisting in 1923 of a simple colorimeter) to waste. By moving the colorimeter along the tube, and knowing the flow rate, Hartridge and Roughton could measure the process after a known time.
In its time this was a revolutionary advance showing an apparently intractable problem (studying a process taking milliseconds with equipment that required seconds for each measurement) could be solved with simple equipment. However, in practice it was limited to reactants available in large quantities: for proteins this effectively limited it to reactions of hemoglobin. For practical purposes this approach is obsolete.