I'm not familiar with the strategy you're describing, but this is not how it works for the majority of backdoor Roth contributors.

If you have $100k pre-tax in a trad IRA, contribute 7k after tax for the purpose of rolling into a Roth, then you will owe income tax on the proportion of 100/107*7k, or $6,542.

You're still limited to 7k annual (for 2025) so the 14k you describe must be something else.