Minecraft wiki entry for Development_versions, associated text: Currently only available through the creative inventory.
Entities, including mobs and players, can enter powder snow blocks.
Their movement is slowed down while inside, and fall damage is ignored.
Powder snow can be climbed similar to ladders or scaffolding. Downward movement cannot be accelerated.
Have a slightly different texture than regular snow blocks.
When the player is inside the block, a powder snow block fog is rendered around them and an overlay texture is rendered on the HUD.
Currently behind Experimental Gameplay toggle (Caves and Cliffs sub-toggle)