Has anyone heard of this? Can you explain?

Formation of a vapor bubble or cavity in a liquid, often caused by uneven pressure. Google it for lots of info.

I thought that was when I had to poop but only farted??

If you are referring to cast bullet defects and it's small cavities, then I understand.
Huge problem with rifle's/Pistols needing a perfect cast bullet that won't wobble out of it's trajectory.
Shot cast 165's in a XP-100 chambered for 300 Herrett for years.

Much better than the reverse ;-)