Presynaptic modulation can be excitatory or inhibitory. It provides a more specific response and more precise control of action potentials. It is specific as the neurone targets specific axon terminals so can inhibit a specific pathway.
Inhibitory presynaptic modulation often involves an inhibitory neurone at an axon terminal. This stops the axon terminal from releasing a neurotransmitter, so no response is triggered from the target cell. There is no neurotransmitter released, as the summation of the action potential from the excitatroy neurone and the inhibitory neurone (at the axon terminal) isn't large enough to cause the vesicles to fuse with the membrane and release the neurotransmitter by exocytosis.