Taxes – underpayment penalty?

Just watched the Taxes video – love the advice. I’ve paid quarterly estimated taxes with the belief that I’d have to pay an “underpayment penalty” if I didn’t (because the IRS wants you to “pay as you go” thru estimated taxes). I’d rather do as you suggested and “pay” that tax money quarterly to my own (interest-earning) savings account, and then pay all my taxes in a lump sum when I do my tax return in April.

How do you avoid getting slapped with an underpayment penalty if you didn’t pay the IRS quarterly?
Or does that only happen if you don’t pay your tax bill in full by the April deadline?