Simplified (not exact) Description:

When you figure your taxes, you have income items, and various deductions, resulting total taxable income. From that, you calculate your total tax liability.

Then you subtract this $7500 tax credit. If negative, your remaining total tax liability becomes zero. The result is your remaining tax liability. You might already have paid more or less than this amount.

So you then subtract the tax payments you already made (W2 withholding, any estimated taxes, etc.). The result, if positive, is the amount you still need to pay. If negative, it is the refund that the IRS owes you.

So, you need sufficient "Tax Liability" to benefit from this special EV "tax credit". The amount of taxes that you still owe (would still need to pay) is not important for this benefit.