I built a mechanism, but if turn off the torch on top, the piston remains active.
After that you can just break mechanism, don't touching the piston.
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Dupe of MC-108

This is a special redstone mechanic called quasiconnectivity. It IS in fact a bug, but so many people started using this bug to make advanced redstone contraptions, that they decided not to fix it because it would break so many redstone contraptions.

I know that such a quasiconnectivity, but the piston is active after the signal interruption.

That is how it works. This is usually called a bud switch.

I never heard of bud switch. It's interesting.