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“The athletes shall be placed in the order in which any part of their
bodies (i.e. torso, as distinguished from the head, neck, arms, legs,
hands or feet) reaches the vertical plane of the nearer edge of the
finish line as defined above.”

This is a quote from the IAAF rules linked to in the article above. It appears that you need more than an outstretched finger to cross the finish line. You need some portion of your torso.

I think this rule may be this way to prevent diving. It would be difficult to get your torso to the finish line any faster by diving…