A piston will activate when a block (in the picture attached it's a iron block but it also works with any other block) is placed on top, while a powered redstone line runs ontop of the block above the placed one. It works with sticky and nonsticky pistons. The dispenser (see picture) will also fire once when the iron block is placed.
I don't think this is meant to be that way.
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This was originally a bug that was then kept in the game due to its popularity and usefulness. The bug is called Quasi-connectivity: https://minecraft.gamepedia.com/Tutorials/Quasi-connectivity
This is WAI. It's called quasi-connectivity.