When having a downward facing piston you can place a redstone block on top of it and a slime block on top of the redstone block. If you move the slime block you will move the redstone block which should cause a block update. Instead it causes the piston to retract and then extend again and you can move the slime block as far away as you want and the piston will stay extended until given a block update.
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As long as bud switches are in the game this is working as intended.
You are correct in stating that moving the redstone block initially causes a block update (which it indeed does) but upon moving the redstone block the piston realizes that it is receiving power diagonally from the redstone block and rextends itself.
However, it has now become a bud switch meaning that you can move (or remove entirely) the redstone block and the piston will remain extended. The piston will remain on indefinitely until you update a block next to the piston.
Torabi
This is a result of MC-108: quasiconnectivity. When you move it one block away, it's still powering the piston, because they check more than adjacent blocks for redstone power. When you move it two blocks away, it no longer is close enough to power it, but it's also no longer close enough to trigger a block update, which would cause the piston to check for power and retract.