See the following figure (source):
The pathway on the right (SN $\rightarrow$ interneuron $\rightarrow$ F Neuron $ \rightarrow $ flexor muscle), is explained as follows:
The action potential in the sensory neuron invades the synaptic terminal of the sensory neuron causing the release of transmitter, and subsequent excitation of the postsynaptic interneuron colored black. This neuron is called an interneuron because it is interposed between one neuron (here the SN) and another neuron (here the MN). The excitation of the interneuron leads to the initiation of an action potential and the subsequent release of transmitter from the presynaptic terminal of the interneuron (black triangle), but for this branch of the circuit, the transmitter leads to an IPSP in the postsynaptic flexor (F) motor neuron (colored red). The functional consequences of this feedforward inhibition it is to decrease the probability of the flexor motor neuron becoming active and producing an inappropriate flexion of the leg.
Specifically, it states that but for this branch of the circuit, the transmitter leads to an IPSP in the postsynaptic flexor (F) motor neuron. But it doesn't give an explanation of how this works.
How can a neuron generate an inhibitory postsynaptic potential when it is given an action potential? Can this specific neuron inverse the signal? How does that work?