When your hand is in this position, the line of vision of the audience strikes the back of it.

2--When your audience is seated so that they can see down into your hand, as in the balcony; or when you have a small audience of two or three very close to you, so that they can look right down on your hand. Shake your hand again to get it relaxed and take Position 1. Now tilt your hand from this position, thumb end toward you, so that you can see the back of your hand at an angle, with your little finger farthest away from you. With this position of the hand, the line of vision of the audience looking down strikes the back of your hand.

3--When your audience is below you and you are on a raised platform, so that they can look up into your hand. Or, if they are seated at close range and you are standing so that they look right up under your hand.

Shake your hand again and take Position 1 naturally. Tilt your hand in the reverse direction from Position 2 --so that you can see the palm of your hand at a slant and your little finger is nearest to you. The line of vision upward of the audience will strike the back of your hand in this position. Remember to practice with both hands, standing in different positions before the mirror. Relax your hands before taking each position so that you will hold them naturally.

When people are very near you, this Angle of Visibility is very important. You must study with the aid of your mirror the amount of slant your hands require in relation to the different places where these people are sitting or standing.
Harlan Tarbell was the mentor of many gnerations of magicians through his famous correspondance course The Original Tarbell Course In Magic