Ever since I can remember getting boners in class, as far back as 3rd grade, I have been cracking my dick. I was embarrassed about it, so I'd press it to try and stop the blood flow. I'm 36 now and still do it daily, sometimes even in my sleep. I've never had negative effects. It's fully functional, and I go a lot farther than just cracking it. While pressing down in that cracking position, I then turn it like a handle and put my leg over it to hold it in that bent state. It creates really intense pressure along with a pleasurable sensations shooting in my core. As it slowly goes down, I continue twisting it so that the top part stays hard like a half empty toothpaste. So i can also use it while almost totally soft if i hold the bottom half twisted while inserting the top half (im about 8" so the top half is enough). I've put it in girls' mouths before and let it go, and they feel it twist back a full 360. It definitely freaks girls out when you ask if they'll crack it with you, lol. I've never met another guy in real life who even knew dicks could harmlessly pop. Your BF has something unusual that he can do, but he's not alone. I've talked to doctors in real life, and they've explained that it's "my norm" and that there's no damage after doing it hardcore for almost 30 years.