I touched the finishing mark and spun round — why didn't my penalty count?

Because the turn had no tack in it. Rule 31 stops you touching a finishing mark while still racing, and Rule 44.2 requires every penalty turn to include one tack and one gybe — going through dead downwind alone is only the gybe. At the finish there's a second trap: your hull must be completely on the course side of the line before you finish again, and starting the turn un-finishes you.

⚡ 30-sec version

<iframe width="560" height="315" src="https://www.youtube.com/embed/KXM-KJoWc_Y" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/v/KXM-KJoWc_Y/">Source: Sailing Kiwi</a></p>