A mono-stable multivibrator is used to create a pulse in response to an external trigger. Why do you need such a complicated circuit for that? Can't we have a simple circuit with resistances and voltage sources. Whenever there is an external trigger something completes the circuit(earlier the circuit was open) and hence current flows through thus creating voltage across the load. Now the circuit automatically opens after sometime(through some mechanism). So this way a pulse is created across the load resistance right?