This is all about previous association - your body reacts to the thing because it has worked before.

The same happens when you take a paracetamol, you always feel better as soon as you've taken it, but it's because previously it did really work.

A similar thing happens with cursing - you can put up with more pain while shouting swear words than normal words, it is believed that is because you associate all those swear words with previously painful/traumatic times in your life and you're kind of drawing on those for extra strength.

The brain is strange and marvellous.