It's not amortized. I.e. the company simply subtracts all salaries for the year from the revenue and pays tax from the difference. Salaries being amortized means on year 1 you can only subtract X% of salaries and the taxable amount becomes much larger. Year 2 you will use the X% for the second year salary and another Y% for the already paid salary of the first year. So, the difference becomes less, and the taxes become less, too.
For a stable company that has a constant revenue stream and an established body of workers there's no much of a difference: instead of paying all tax for current year salary you pay 6 chunks of tax for 6 different years of salaries - which would be about the same amount.
For early companies things can be pretty tough. You may earn, say 100k in a year one and pay your employee 100k. Your company now has 0 in the bank, but for the taxation purposes the taxable amount is like (100k - 10% of 100k = 90k), at 20% corporate tax that would mean that the company has 0 in the bank but owes the government $18k in taxes. It's much harder to start a software business in this kind of environment.