it's exactly like the freelancer tax brackets in Italy. Everybody jumping through hoops (and or not declaring stuff) to remain under the threshold of 60k/year (was 30k/year). Otherwise tax rate doubles.
[1] https://en.wikipedia.org/wiki/Progressive_tax (See the Computation section for how the higher rates only apply to the higher portions of a person's income, with their lower portions taxed less.)
0-9000€ [0%] -> 0€
9000-20.000€ [10%] -> 1100€
20.000-40.000€ [15%] -> 3000€
Total = 4400€
So if you're in the 15% bracket you pay 4400€ which is actually 12.5% in total, and not 5250€ (15%) as some people seem to believe.