My CBT therapist taught it to me, recently when ive been going to my lessons ive been worried about fainting cause of the physical symptoms i feel from panic. He got me to write down all the physical symptoms of panic and then compare them to the physical symptoms of fainting, so i wrote it all and compared the two, and there was only one similarity between the two. 

Just writing down the symptoms and seeing the facts, helped alot to rationalise that all i was feeling was panic and nothing more, it helped alot with my health anxiety