Ejaculation
'Shooting your load' is what people casually call a guy's climax, specifically the ejaculation. The stuff that comes out, the ejaculate, is made up of sperm and secretions from the prostate, and it's produced in a bloke's testicles. Sperm is made fresh every day, and the body naturally gets rid of the unused ones, often through wet dreams at night. It takes about 72 hours for a sperm to fully mature. Because of the daily production, a man's testicles always have sperm cells at various stages of maturity. Healthy blokes typically shoot out about 1.5 ml to 6 ml of sperm each time they climax.
The amount varies, depending on how long it's been since the last time and the guy's age too. Men around 30 have the highest sperm production. But, the amount of jizz has nothing to do with a guy's fertility. Stress, certain underlying health conditions, medications, and hormonal imbalances can affect the amount too. Generally, the shorter the time between climaxes, the less you shoot. Most of the sperm comes out during an orgasm, with the prostate and the vas deferens contracting hard, which flings the stored ejaculate out. A bit of sperm also comes out when a guy's turned on. This pre-cum's got a fair few sperm in it too. Strong pelvic floor muscles, especially the pubococcygeus muscle or PC muscle, can help a bloke hold back from ejaculating. This holding back is called 'injaculation'. The sperm gets reabsorbed into the body after orgasm. It's not harmful or anything. Blokes can also stop themselves from climaxing by pressing the Jen-Mo point, between the bum and the balls. But, a lot of guys prefer to shoot their load in or on their partner.