Typically you get a signing bonus that vests of a period of X years. Once those X years are done, you are relying on additional stock grants from past performance reviews or promotions to keep your comp at its previous value.

If you perform well or get promoted you should see your compensation increase, but without that you are left with your base salary and no additional stock grants and can take a pay cut as a result.

Its one way FAANG weeds out lower performing engineers.