"Falling blocks" refers to sand, red sand, and gravel.

Blocks that will prevent falling when directly underneath the placed falling block are: skulls, torches, slabs, switches, redstone, etc. – basically anything that doesn't have a valid top surface for falling blocks to land on

To reproduce, simply place a block without a valid top surface to catch the falling block when it's dropped from a higher point, then place a falling block directly on top. Rather than falling, the block will simply sit where it was placed