OP, you might want to spell out the concept in case anyone on the forum doesn't fully understand it either.
I'm not an accountant but my understanding is that if you fall in the 70% tax bracket or whatever, only the income over the cutoff point is taxed at that rate. So if you earned $100,500 and the cutoff was $100,000 you would be taxed 70% on $500.
At that point I'm unsure how it works. I would guess it keeps chaining down, but for all I know everything else you earn (the $100,000) might be taxed at the next lower rate. Someone who knows tax codes well please correct me.
Also as someone pointed out, earning more money may also affect which deductions you are eligible for, which complicates matters.