The word for this kind of vesting schedule is "golden handcuffs," because once you cross the threshold of two years, it would be irrational to leave for somewhere else. Amazon is paying you in year 4 for value you delivered in years 1, 2, and 3 (otherwise you wouldn't make it to year 4). It works well for Amazon because they only pay you lots of money once they know you're a good engineer, and because you have an increasing incentive to stay each year.