Sounding is sliding ( usually) a perfectly smooth polished and round bar of surgical steel up your urethra.
Lubed of course. And sliding it in and out as you masturbate or are masturbated.
Generally, it involves starting with a slender sound, and gradually working your way up to ever thicker sounds to stretch out your urethra, and for more intense stimulation. You can sound just the length of your dick- or further- all the way up into your bladder if you have long enough sounds with the necessary bend in them.
They are medical devices used by urologists to try and open clogged, scared or otherwise damaged urethras... but used sexually, they can be amazingly hot and pleasurable.
At first use, it will feel painful or very uncomfortable to stick something up your urethra... but that is mostly just nerves and bad technique. Once you get use to it... its purely pleasurable and you only feel mild pain or discomfort when you size up.
Some guys get carried away and keep on stretching their urethras until they can stick index fingers and larger objects all the way up their dicks.
I spent about 4 years sounding and loved it and would recommend it highly to those curious. However, the one thing to be aware of is extremely good hygienic practice- I sterilized my sounds before and after every use... the last thing you want to do is shove any kind of bacteria way you your urinary tract.
I've never really been curious about it... but the way you posted made my brain tingle a bit...
You get me thinking Phil.