Can you teach me how this tax avoidance works? It seems that I am missing out. Can you also specify the jurisdiction where this works?
> mostly part of their contract
I can think of many people whose bonuses are 3x or more their salary and who do not have a guaranteed minimum bonus or even a strict formula based bonus.
This only affects the withholding of bonuses (the amount the IRS has you pre-pay as an estimate of taxes.) It does not affect the actual income tax on bonuses.
(It's mostly a disadvantage. I get about 60-70% of my income in bonus and stock which is treated the same; it just means that, if I don't plan ahead for it, I end up with a surprise tax bill on April 15th. I do plan ahead, but the whole process is obnoxious.)
There are plenty of games you can play with withholdings. EX: As long as you witholdings are enough to pay last years taxes or 90% of this years you are good.
Or if your bonus is in January you can have a higher witholdings from salary and the default 25% from the bonus. Just balance it by the end of the year.
But that's just simple stuff, there are also other benefits to the company.