The stiff-finger or knuckle attack in the sand. It exists only because the FIVB beach rules outlaw the soft open-fingertip lob.
Indoors you are free to tip the ball over the block with your fingertips — in the sand that same touch costs you the point. The FIVB beach rules allow only a clean hit, the heel of the hand, or the poke: stiff fingers or knuckles, a short hard contact, no soft guiding of the ball. The reasoning is practical: on the beach a referee decides without slow motion, and open fingers are far harder to judge cleanly than an unmistakable jab. The classic switcher's error is the reflex fingertip tip the moment the block goes up — which also happens to be a good way to jam a finger. So treat the poke as a technique in its own right, not as an emergency option. And keep the two situations apart: on defence the open hand is only legal against hard-driven balls, while everything soft is taken with the forearms or a tomahawk.