You put it on. Then suddenly you feel an emptiness in your crotch. You get the feeling that things are reshaped--and when you lift the jock strap to see underneath it, you realize you now have a vagina and a clit. The only reason you still felt anything at all is that some of the nerves from your male parts are still there but now going to your new female parts.
You wonder just why in the world the jockstrap, even a magic jockstrap, would do such a ridiculous thing. Someone who wanted to be a girl wouldn't wear a jockstrap--it's a guy's clothes, after all.
You open a small booklet that's supplied with the jockstrap; dimly, you realize that you probably should have done that first before putting strange magical devices on your body.
"Thank you for using the QZX-8000 series of products. The QZX-8000 series is designed in the form of an ordinary piece of clothing and is used to deliver targeted magical spells to the specified area of the body. Versions available are the QZX-8000 (shirt), QZX-8001 (jockstrap), QZX-8002 (gloves)...."
You figure it does make a little sense. If there was a spell to change your genitals, it would have to be placed on something that touches your genitals. Thus, a jockstrap. You wish that they could have put it on, say, women's panties instead.
But it's too late for that. What can you do?