Each element in the vector is first multiplied by the forgetting factor |forget_factor_|. Then the vector element indicated by |iat_packets| is then increased (additive) by 1 - |forget_factor_|. This way, the probability of |value| is slightly increased, while the sum of the histogram remains constant (=1). Due to inaccuracies in the fixed-point arithmetic, the histogram may no longer sum up to 1