This is because after the completion of reaction it gives pinkish colour which indicates that the reaction is complete As we all know that the potassium permanganate (KMnO4) is the dark purple color solution and in titration it become colorless. So when the titration is completed, the other single drop of KMnO4 solution produces pinkish colour in the beaker which indicates that the reaction was completed just a single drop before .. 
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