The aroma of Szechuan peppercorn is similar to that of lavender. When consumed, the first taste is bitter, then a numbing heat is sensed, followed by citrus flavour. This spice is famous for the powerful numbing sensation it causes around the mouth.
These peppercorns contain the molecule hydroxy-alpha-sanshool which interacts with nerve cell receptors in the lips and mouth.
It excites the touch sensors and the confusing sensation feels like numbness. With enough stimulation, it feels like the area has gone numb.