As a Q&D example, if DH's and my gross income is $50k and have 3kids we claim, and no payroll deductions are made, line 7 is $50k, EITC on that is about $325. If we contribute to tIRAs $11k, our AGI is $39k, and the EITC on that is $2641. Since our credit is whichever amount is smaller, we get $325.

If DH maxes his 401k, our line 7 wages are $32500, the EITC on that is $4010. Our AGI is pretty much the same (add a little interest, subtract the teacher $250), so the EITC is the same. If we again add the tIRA, our AGI would be $21500, and the EITC on that is $6044, the max. However, the credit on our wages is smaller, so we only get the $4010.

For our simple tax return, contributing to tIRAs won't increase our EITC, but payroll deductions help enormously.