The event is not cancelable.

The event does not have a result.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.forge.api.event.block.piston.PistonEvent;

PistonEvent extends BlockEvent. That means all methods available in BlockEvent are also available in PistonEvent

Name	Type	Has Getter	Has Setter	Description
Name direction	Type Direction	Has Getter true	Has Setter false	Description Gets the direction that the piston is facing.
Name faceOffsetPos	Type BlockPos	Has Getter true	Has Setter false	Description Gets the position that the piston is facing towards.
Name level	Type Level	Has Getter true	Has Setter false	Description ​
Name pistonMoveType	Type PistonMoveType	Has Getter true	Has Setter false	Description Gets the move type of the piston (is it extending or retracting).
Name structureHelper	Type PistonStructureResolver?	Has Getter true	Has Setter false	Description Gets a nullable structure resolver that can be used to get all the blocks that will be affected by the piston. Be sure to call the resolve() method on the structure resolver