It's an intake that is open at the top/center of the hood and funnels cool air into the stock airbox. There were several threads, just a week or so ago, here in this section. Also, MM&FF and 5.0 magazines have done some lengthy write-ups in recent months and BON (blueovalnews.com) has some info/pics...
Classic Design Concepts created the shaker, and it is available for any '94-up Mustang, BTW.
The old style like on the Trans Ams of yesteryear is basically the air cleaner box sticks up through a hole in the hood and draws fresh air from the outside. Because the box or scoop is attached directly to the engine it "shakes" or moves as the engine does when idling rough or under hard acceleration hence the name "shaker hood scoop". Kinda cool intimidation factor.