The pistons head (the extending part) counts as a redstone block.
If a pressure plate is placed and a piston put one adjacent block below, when activated the piston head will extend but not retract due it now being level with the pressure plate and receiving a redstone signal keeping the entire piston lit.
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The bug you are pointing me to says "works as intended" do you know why Mojang would leave something like this in?
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There is quite a long discussion about it in MC-108, with official statements from Mojang.
A picture is worth a thousand words, but I think you mean MC-108.