I don't know what to call it. But it's some kind of invisible summing of block thing, where when they get stacked on each other, the top 1 becomes invisible.
See This Video: https://www.youtube.com/watch?v=yM8uT-isGis

(I Did it with creepers cause I felt like it, you can use this command too: )

/summon FallingSand ~3 ~2 ~ {Time:1,Block:"stone"}

summon it until it has reached the top of stack, then summon 1 more time, the block will drop, but the next block will be invisible.