> Maybe this is a dumb question, but if you only deduct part of their salary in the first year, what happens if you have a software developer for several years?

Not dumb at all! In the second year, you get to deduct ⅕th of the previous year’s salary and ⅕th of the current year’s salary; likewise, in the third year you get to deduct ⅕th of the first year’s salary, ⅕th of the second year’s salary and ⅕th of the third year’s salary.

The key thing is that in the fifth and following years, a business would deduct a fifth of each of the previous five year’s engineering payrolls. This is not great for a growing business, but it’s murder on a startup trying to grow from zero.