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A slider will appear on the left side, to allow letting you to select which blocks should run:
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Note that the first block in your selection will reuse the output from the previous block in the chain.
In the screenshot the Mapper block would use the last output returned by the “GetEmployees” block, as shown in its Response tab.
If the previous block in the chain has an empty Response tab you may need to re-run it (or other blocks further up the chain), so that the blocks in your Special run will have input data to use.