Picture this in your mind. You and your friend are enjoying a nice quiet chess game. Suddenly you sacrifice your Queen and before he has a chance to close his mouth you attack him. This is the nature of this type of sacrifice, unexpected and vicious, especially since the books and other materials available about thematic sacrifices do not usually list this one. This is, of course, a sacrifice done by Black. But White can get in on the fun by sacrificinging his Queen at "g6." And keep in mind that a Queen is not the only piece that can be sacrificed. A Rook, Bishop or Knight would do as well.   Do unto others before they do it to you! Here's a mini game that illustrates:

There are many more examples of this thematic sacrifice here