In ASA, the fielder is not allowed to impede the progress of the runner (including blocking the base) unless the fielder has possession of the ball (or is about to receive the ball - defined as the ball being closer to the fielder than the runner) and is attempting to make a play on the runner.
A fielder anticipating a throw, or even a ball in flight, does not relieve the fielder from the obligation to not impede the runner.
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Tom
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