It's hard to describe. But it's kinda like if you've ever gotten a real bad bruise on a muscle you use all the time (not that we use balls all the time but they're constantly moving/rubbing against thighs and such as we walk/move).

Like the area just feels heavy and swollen and sore and stiff. Light touches can be a bit painful. It's hard to ignore. But that said, obligatory: it's not harmful, and guys can cure it themselves easily enough by jacking off so never listen to a guy using that as an excuse to pressure you for sex.

And even though it does happen, it's not common. At least not for me.

Potential TMI upcoming to illustrate point. Like the only time I've ever gotten them was first when I was a teenager and really made out with a girl for the first time. Like at 15 that level of excitement and hormones meant I was at full mast and ready to blow pretty much the entire time I was with her. Like we probably spent an hour just heavily making out. Walked away with blue balls not even knowing what they were at that time.

Since then the only other times it's happened has been when I've had prolonged sexual stimulation without actually finishing. Like I had a girlfriend once who used to like to tease and have extended foreplay sometimes in "inappropriate" places where going further isn't an option. So once in a while it would happen then.

But yeah, it's not like it happens from just getting a boner anymore.