Often used for strength exercises, mixed grip is useful while descending stairs in order to reduce risk of injury.
Descending stairs without handrails use is with no doubt the less safe solution and having at least three (1 hand+2 feet) or four (2 hands+2 feet) points of contact, even if it provides more safety and balance, is not synonym of wrists injury and falling forward risk prevention.
Mixed grip technique named “trailing hand technique” is effective while descending narrow and steep stairs in addition to a difficult handrail grip. It consists in trailing your “strongest” hand with palm up underneath the handrail behind you forcing your trunk twisting slightly (subsequently feet).
Result? Stronger and responsive wrist, controlled descent and larger walkable surface on step…walk like an egyptian…
See attached picture.