It’s when they don’t make the air cell and try to external pip instead but can’t break the shell. At that time you immediately make a tiny pin hole in the middle of the bruise then wait 48-72 hours
With twins they often can’t turn to make air cell
If the lower one does the higher up one will...
I tried twins and got them to hatch but lost them during hatching as you have to watch very close every few hours I missed the bruising while sleeping.
You can check my post about it and see if it looks like yours
At day 10 you should see 4 circles
Each baby has 2
One is the head and other...