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If you set up a button, a piston and a redstone dust in this composition...

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The piston will be activated while the button is on (and will retract when off).

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However, I can't find the rationale why this gadget works. I guess this is some kind of BUD mechanism, but still don't understand what route the piston is powered through.

As far as I know, if I remove the dust below the button, it'll no longer trigger the piston.

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Nor when the dust is powered otherly.

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  • @ppperry Thanks, that post doesn't particularly answer me but the article they cite does. It seems that the chunk of information added to that article in last year's July provides the clear explanation why my formation works. Commented Aug 11, 2018 at 10:40

2 Answers 2

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This is a block update phenomenon. The button is providing diagonal (BUD) power to the piston, and the redstone dust is providing updates to the piston, which causes it to extend. Normally, block updates are only triggered on the 5 sides of a piston. However, redstone updates are special, and can provide updates to a piston 2 blocks away.

The dust doesn't provide any power to the piston (which is why the piston doesn't extend with just the dust in this configuration), however, it does provide a necessary update to the piston (which is why the piston doesn't extend without it).

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  • Do you mean redstone updates also affect blocks in diagonal positions? Commented Feb 10, 2017 at 16:38
  • I believe it's a 2 block radius, particular to redstone updates (including placing redstone. Block updates are weird)
    – Unionhawk
    Commented Feb 10, 2017 at 18:31
  • Thanks. I tried a bit and I think I understand that at least redstone torch and dust act the same way on update. Do you know any source referring to this behavior? Commented Feb 10, 2017 at 20:00
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This is called quasi-connectivity, it is a property that is shared by pistons, droppers and dispensers. Basically, what it means is that the redstone appliances that are effected by this phenomenon can be activated by having the block above them powered, this includes air blocks. This may seem like a bug, but the team at Mojang have stated that it is a feature that works as intended. It sometimes causes trouble in circuitry, however it is the premise that many piston doors work on. Without this feature, flush piston doors would be incredibly complex and difficult to build.

You can find more information on the subject, as well as tutorials, on the Minecraft Wiki.

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  • Yeah, I think this "feature" will be going away at some point. It most certainly is a bug, but it isn't necessarily easy to fix without breaking a bunch of other things.
    – MBraedley
    Commented Feb 10, 2017 at 18:47
  • @MBraedley this is not the case, this was intentionally added by the developers, and is essential in many contraptions. It is not going to be removed Commented Feb 10, 2017 at 19:02
  • Cite your sources. It was introduced because it fixed another redstone bug, and was kept because it was more desirable than the bug it fixed.
    – MBraedley
    Commented Feb 10, 2017 at 19:06
  • The developers have stated "It works as intended" Commented Feb 10, 2017 at 19:09
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    A wiki is not generally a citeable source, since they're globally editable and anyone can add unverified text. In this case the wiki itself had a reference to the actual bug with Mojang. I've edited that into the answer to make it more standalone. Commented Feb 10, 2017 at 21:28

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