I've seen this hand sign referred to as both the sign of blessing, and as the sign of esotericism. Both examples are used in the Tarot. As a sign of blessing the palm is out; as a sign of esotericism the palm is concealed.
The origin of the sign used in the RWS comes from the ancient Romans, from whom the Roman Catholic Church adopted it. It was originally known as the hand of Jupiter, and was used to avert the evil eye. It had three raised fingers and two lowered. If you look carefully at all three cards mentioned from the RWS deck, you can see that three fingers are actually raised - the thumb, index finger, and middle finger - and the last two are lowered. From the church's standpoint, the three upraised fingers represent the 3(Trinity) which triumphs over the 2(Duality).
-fof