As Shane has said, it's a hand gesture that was used for a solemn oath. You normally say "ich schwöre"; I haven't seen this used for a mere promise. If you watch for example when Bundeskanzlerin Merkel was sworn into office, she made a similar gesture by raising her hand, except that the typical finger position seems to have gone out of fashion with current politicians. :-) If you do this gesture and cross your fingers, it means that the oath is not sincere and not valid. This variant of the gesture is still in use, even for promises, often done behind your back.

Ich weiss kein deutsches, kulturelles Symbol, das auf diesem Zeichen beruht. Ich vermute mal, es handelt sich um eine Variation des Victory- oder Peace-Zeichens. Oder eine Anspiel auf "Versprochen" (I promise) wo das "V" für das Zeichen steht.

http://en.wikipedia.org/wiki/Schwurhand