Underberg's recipe (now over 160 years old) is a closely guarded secret. Created as an after meal digestive aid, the syrupy liquid (which comes in the same awesome little “portion sized bottle” as it did when it was first introduced) is super strong tasting; it overwhelms the mouth with complex spices – with anise as a dominant flavor.
Meant to relieve tummies after meals, I found (and maybe it was just psychological) that it did in fact make me feel less bloated after my Peter Luger burger and onion rings. Centuries of Germans have sworn by it.
The bitters can also be used in several little known cocktails detailed here.