Put your hand like this: https://static1.bigstockphoto.com/9/5/2/large1500/259859530....

But with your thumbs closing your ear orifices.

Now you have to hit the back of the neck with the tip of your middle fingers, and to get a harder hit you "snap" the fingers, putting the finger like "fingers crossed" position, and the pressing the middle finger towards the head. You should hear a big "thump!" inside your head.

It aleviates the tinnitus for a few seconds, but more likely due to the stapedial reflex than anything related to neck muscles.