It’s the same concept. The first two years you are getting your base + prorated signing bonus + 5% of your RSUs the first year and 15% the second year. If the stock price stays level. Your 4 year compensation is going to stay about level - you get a slightly smaller signing bonus the second year. If it goes up significantly by year 2.5 (20% every six months after year 2). You will see a massive jump.

If Amazon did just pay you base + stock at an even 25/25/25/25. You would still be “leaving money on the table”.