It's physics... In case people are wondering. A cup can stop and 8 oz ball of a 4 oz puck from wrecking your boys if you get a shot. There isn't enough mass from those object to case serious injuries to the joint. However, when you're talking about 300lbs man hitting you with his helmet or shoulder pad, the force placed on the edges of the cup, which sits right against the crest where the hip joint fits into the pelvis can force said edge into this area and actuely pry it out!!!
If you all recall, there was a player a while back who was accused of grabing guys crotches in games... Again, he wasn't wearing a cup because this is the norm in this sport.
Thanks to VIPER!