sounding
Urethral sounding is the medical use of probes called sounds to increase the inner diameter of the urethra and to locate obstructions in it. Sounds are also used to stretch the urethra in order to receive piercing. Urethral sounding and urethral play are also used to refer to this practice in a sexual context. Urethral play can involve the introduction of either soft or rigid items into the meatus of the penis (as well as farther in). Objects such as sounds are usually only inserted about halfway into the glans and can usually be easily retrieved.
I love watching a sounding rod disappear one cm at a time into my sub's cock. I find this kind of intrusive play very exciting.