What is a jock strap and how is it used? I have never heard of it.
A jock strap or athletic supporter is primarily used to protect your testicles during athletic activity. It keeps the testicles pressed up against the body and keeps the testicles from twisting as you move quickly or when you accidentally get hit. For it to work, it must be the first thing you put on, going on underneath your underwear.
Some guys find jock straps to be uncomfortable. Recently, clothing manufacturers have come out with compression shorts. These work the same way as a jock strap, but also serve as replacements for underwear at the same time.
Neither the jock strap or compression short can protect you from a direct blow to the groin. For sports where this might happen you can get either style with a pocket over the genitals where a protective cup is inserted. The cup is made of hard plastic that covers the genitals and redirects any blows to the edges of the cup, which has padding to soften the blow.