Basically, Urethral Sounds are medical devices (used to widen the urethra) they are are inserted into the urethra to (depending on what kind of sound used) "stimulate".
these curved ones go in (while limp only) and are to be guided down urethral tube to bladder, to stimulate prostate (not go inside bladder).
These straight ones can do same thing but require that the penis be moved alot and that takes someone very skillful person to not get hurt. but i think straight ones are just for pleasure alone.

I personally am interested in this. It is inserted and can vibrate inside.

But even though the web says its safe if you know what your doing, i want to make sure its safe to use so my Girlfriend might decide to do this with me.
On a side note? is this a very weird fetish?