What does "plays" really count in iTunes? iTunes displays the number of "plays" for each track. What exactly does this mean?

For example, playing a little bit of a track does not seem to increment the number. But playing most, but not all, of a track seems to.

Does playing n seconds trigger the increment? n percent of the track? Or something else?

I have a 4GB iPod Mini, if that matters.

The motivation is, I have iTunes set to automatically delete podcasts after they're played, and I don't want to accidentally trigger a deletion.
I believe that you just have to listen to the last few seconds of a song. You can skip all of the beginning part, and it will increment the Plays if you listen to the end.