What is an internal pip?

[tsid78]

Hi there,
I've tried to search the forum for this but can't seem to find it. When candling a duck egg (mine are on day 25 tom and about to go into lockdown), how can I tell if there is an internal pip??

 

[Pyxis]

It's when they pip into the air cell. When candling, you'll see the beak as a shadow in the air cell.