So many low-cut and ultra-low cut jocks and styles of swimwear are on the market, keeping junior tucked inside becomes a challenge especially when he gets fired up and wants to take a peek at the outside world. What I've found works for me is to sew a tiny piece of elastic just inside the top seam and tuck my friend underneath. This inevitably provides a bit of stimulation, which produces a better bulge and maybe a little more exposure than intended, but usually prevents total pop-out.