It does not come off your schedule C so you still pay the 15.3% SE tax on the Net Profit. It is an adjustment to regular income on Schedule1 line 16 which goes to 1040 line 10. It only reduces your taxable income on 1040 line 15.
And having less taxable income and less tax on line 16 can reduce any credits you were getting. You need to compare 1040 lines 16-22 before and after you enter the 10,000 401K.
