When you have them by the balls, literally!
Squeezing, slapping, wrapping, tying, binding and other types of Cock and Ball Torture (CBT) are often a serious test of dominance and submission.
Cocks can bruise and blood vessels can be damaged during CBT. If you see some bruising and it’s just below the skin, it will probably go away in a short time. If the bruising doesn’t go away or appears to be growing beneath the skin, see a medical professional. It’s difficult to “break” a penis, but if you should strike an erect penis and then see that it can be folded over (as in half), you should stop play and seek medical help.
Balls are incredibly resilient and can take more torture than you might imagine, but can be damaged by severe blows. If you’re going to do a lot of ball torture, learn about ball anatomy.
The piss channel is quite fragile and can get irritated or even infected if you’re using a urethral sound or catheter. Bladder infections are no fun. Even with proper sterilization procedures, many bottoms experience irritation for a day or two afterwards and are usually encouraged to drink plenty of cranberry juice for healing. Never put anything sharp or breakable into the urethra. If you’re using sounds or catheters, it’s best to learn from an experienced player first.
Finally, having something in your piss hole during an orgasm carries a risk of infection and damage. If your bottom’s going to cum, it’s best to get the sound or other object out.