I didn't even know this was a feature until I looked in the code.

Apparently, if you run out of sats, you can still post limited visibility posts:

function FreebieDialog () {
  return (
    <>
      <div className='fw-bold'>you don't have enough sats, so this one is on us</div>
      <ul className='mt-2'>
        <li>Free items have limited visibility until other stackers zap them.</li>
        <li>To get fully visibile right away, fund your account with a few sats or earn some on Stacker News.</li>
      </ul>
    </>
  )
}