is a form of neutral greeting on Instar
How to pass hands
The hand is loosely curled into something resembling, but not mistakable for, a fist. The arm is then extended such that outside of the hand (i.e. the little finger side, not the thumb side) brushes along the outside of the other person's hand in one smooth movement. This is done in a diagonal direction, not directly towards the other person's body.
Choice of hand
The choice of hands matters a great deal and is highly dependent on the situation, in accordance with the alternating ascension
principle. The hand you pass with displays your status or position relative to the other person. Using the wrong hand is either a catastrophic blunder or a killer snub.
An anecdote often used to demonstrate to aliens the importance of this is of a certain Virtue Minister, famous for his taciturnity, who promoted an underling with a pass of the hand. A point of interest is that the Minister could equally well have demoted the underling using the same hand.