It's more intuitive to think of a cohort than an individual. Lets there are 1000000 lives at the beginning of the year with lets say q' = .02. The lives are dying at the rate of 20000/year with the number of deaths in any time interval proportional to the time interval. Over the year the number of survivors is decreasing by the force of mortality increases exactly enough to keep the rate of deaths constant.
UDD is equivalent to the product of the probability of survival and the force of mortality is constant.
