Obstructed Runner on an Out

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I know this has got to be in the book somewhere but I can't seem to find it.

BR hits a long fly ball to the outfield. He hustles all the way around first and is making an attempt for second. He is obviously obstructed by the firstbaseman. The ball however is caught in the outfield for the out.

What is the proper mechanic here? Delayed dead ball with the left arm, then an out signal, or simply nothing after the catch? Does the delayed dead ball need to be explained (or waived off if need be) with any other signal?