Here is a video of the problem:

(Only works with sticky pistons)

 When sand is on an extended piston head, and then you retract the piston, the sand doesn't fall. When you let the piston extend and then retract at a fast speed, the piston extending will update the sand, and then the retraction will allow the sand to fall down, if you move the piston fast enough.

https://drive.google.com/open?id=1ygSVlsnmd4ubJMNfezBIse6C-K2AEPHS