We often hear the instruction to spread the fingers and thumbs as wide as we can in Adho Mukha Svanasana (Downward Facing Dog Pose) and other hand support asanas.
The idea behind spreading the fingers and thumbs as wide as possible is to establish a more firm and balanced foundation in the hands. A firm (yet resilient) and balanced foundation in the hands is important in reducing potentially injurious pressure in the wrists, and it's also important for stability in arm balances.
However, with the thumb stretched as far as one can place it from the index fiinger (abducted), we overstretch the commissural ligament that traverses the traingular thenar space between the thumb index finger. We can also place undue pressure on the palmar cutaneous branch of the median nerve.
In practicing and teaching hand support poses, we want to emphasize spreading the fingers as wide as one comfortably can, the thumbs not so wide. Reduce the space between the thumbs and index fingers by about one-third.
This alignment 1) allows more firm rooting of the knuckles of the index fingers and thus a more stable foundation, 2) helps balance pressure across the entire span of the hand, and 3) reduces pressure on the nerve strands and ligaments in the thenar space.