The second cause is pressing the index finger hard against the arrow while it rests on the string.
The third is the strength of the bow contrasted with the weakness of the archer.

The shaft wobbles at the outset because the two siyyat—being warped—deliver two unequal pushes; the arrow therefore veers under their influence. Once it reaches the middle of its range, the defect abates and the shaft straightens (1).

Likewise, when the index finger is pressed excessively against the arrow, the shaft bends while still on the bow, especially if the slot of the nock is wide. As soon as the arrow leaves the bow, however, it resumes a level course; the flaw diminishes and the arrow flies straight.

The same applies when a powerful bow is paired with a weak archer: numerous faults arise. Let the archer beware of shooting with a bow beyond his capacity, for that mismatch multiplies his errors, lessens the damage he inflicts, and brings upon him many personal shortcomings. Among craftsmen, a sign of an archer’s full mastery is that he chooses a bow slightly below his own strength.

Section
On the thumb’s being cut by the arrow during the draw and its remedy

The wound appears in three places:
The first is at the thumb-knuckle (2);
The second is at the joint located at the base of the thumb.