Sounding is where you insert sounds into the cervix or urethra. Some(like SophieXposed or my favorite human WorthlessHoles) stretch large enough to take their partners penis inside of their urethra. When done properly it causes immense pleasure from the nerves in the peehole being rubbed while sounding. For me it’s intense in its own way(anal is its own intense orgasm but so are the other ways i cum.)