One hand palm upwards
Gesture of recipient, or asking for something.

Both hands palm upwards
You are at the service/command of whom you are talking to.

One hand palm forwards
Gesture of prohibition, i.e. Halt! Silence! This would never be used in greeting.

One hand palm backwards
Greetings, welcome.

One hand edge forwards
Casual greeting in passing, no further speech desired.

Both arms wide, below shoulder level, palms upward
No weapon in either hand (I come in peace).

Extending the fingers for any of these increases the significance of the gesture

(Paraphrased from an essay in Vinyar Tengwar).