When you have a trusted lover, the fun can really start. Mine has inserted lubricated 6.5 mm metal sound 23 cm in. This is done before I get hard. She pulls my penis out around the sound and it slides in slowly and effortlessly. At about 20 cm the sound passes into the prostate (sweet spot) accompanied by a hot rush of arousal and rapid rise to a very strong orgasm. I draw out the sound during ejaculation to make room for the load because 6.5 mm is a snug fit but others come by narrower sounds. This can be very exciting and may become more mainstream over time.