Navel stimulation is the act of straight up fucking someone in their abdominal pussy, which we human beings call it either the belly button or the navel. According to some sources ( i haven't tried doing it to a lass because i'm too much of a virgin and self control is a thing ), women actually find the navel as a rather sensitive and pleasing spot, so i guess you wouldn't be the only one who's gonna get batshit in heat ( unless you're taking the example of nnS, that will be first degree murder ).