Okay, I'm going preface this with saying that I'm NOT a medical professional, so take everything I say with a grain of salt and double check. However, I am a sadomasochist with a rather high pain-tolerance, so I actually looked into general safety tips for this.
Here's how the mechanics of ballbusting works: getting hit in the testicles hurts. A LOT. I doesn't matter how tough you are, getting kicked in the balls will send you in a world of pain. For illustration purposes, here's
a black bear hitting himself in the nuts while trying to dislodge a water-tank.
But, while it very much hurts, you balls are actually surprisingly well protected in their natural state. There's plenty of room inside, so if your testicles get hit, they can be pushed around inside your ballsack, which will dissipate most of the force from the hit, preventing actual damage. Not a perfect analogy, but imagine putting some water in a deflated balloon, then dropping a ball inside it. Hitting the balloon won't do much damage to the ball in it, since it can just move around. So punching and slapping, as long as you don't go ridiculous extremes (e.g. using baseball bat with full force) is in general safe. Will hurt like motherfucker, but highly unlikely to do any actual damage. Using devices to "crush" your balls (so, applying constant pressure from multiple sides), CAN pose actual threat if you are not careful. That's why I'm not a huge fan of parachutes or similar devices: is you screw up and it slips on your balls, crushing them together. that can cause real damage.
Now, this whole thing above is only true for unrestrained testicles. If you tied yours up tightly, be a LOT more careful: the main "defense mechanic" is that your testicles can move around in your ballsack. With that not being the case anymore, hits on your tied up balls will directly hit your testicles, the thin layer of skin and nerve endings won't do much to mitigate that. hits that would be more or less perfectly safe on an unrestrained pair of testicles could become legit health risk. if you look at the pro porn videos, you will notice that whenever a domina does something flashy, like full on kicking the guy in the ball so he keels over in pain, the subs balls are never restrained. Because that force on tightly tied up balls will very quickly make you an eunuch.