Fingerport Left Hand Rest

The flute has a circular tube. It is the source of instability. Try to lay a pipe on the floor and see how easily it rolls with any external force… The Fingerport is very simple – it provides a large flat surface for the left index finger to support the flute.

Fingerport-Theory Behind Holding/Balancing the Flute
The Fingerport allows greater balance in holding the flute. It is comprised of a C-shaped shell and a thumb-like extension. The Fingerport helps the player to place the index finger at an efficient spot and provides additional support.

Analysis

Mechanically, there are 3 factors associated with balancing a flute:
1. the mass (weight) of the flute,
2. horizontal rotation,
3. axial rotation.
The gravity center of a flute is located slightly to the right of the left index finger (see letter G in Illustration 1 below). When the knuckle of the left index finger takes up most of the weight of a flute at number 2, the small remaining weight is often assumed by the right thumb.
When playing a flute, the player pushes the embouchure plate against the chin (Ill. 1, number 1). With the left index finger both supporting and pushing, the flute might rotate clockwise without a counter force from the right hand.

Illustration 1 

Additionally, the uneven weight of the flute mechanism contributes to another rolling tendency. This can be seen in Ill. 2. The flute rolls backwards toward the player.