Original: The chirp (which only male crickets can do) is generated by raising the left hind leg to a 45 degree angle and rubbing it against the right hind leg edge of the right hind leg, which has a thick scraper (Berenbaum 1995).
Simplified: The chirp is made by raising their left forewing to a 45 degree angle and rubbing it against the upper hind edge of the right forewing, which has a thick scraper (Berenbaum 1995).