self employment tax penalty - when is it assessed? at time of filing or after?

The IRS employs a pay-as-you-go system. When you don't make quarterly estimated tax payments for your self-employment income, the IRS assesses a penalty for underpayment of estimated tax. Here's how it works:

• If you are responsible for an underpayment penalty, TurboTaxwill automatically compute and add that amount to your return, and reduce any refund or increase your tax liability as applicable.  
• To Enter/review Underpayment Penalty here.
  1. Open your return (if not already open).
  2. Enter "underpayment penalty"  in the Search box.
     Click on "Jump to underpayment penalty" link.d

 TurboTax - Does TurboTax Automatically Add Underpayment Penalties