Here is a link to a video by Thompson Owens from Sweet Maria’s on first and second crack during roasting. He has some slow motion video showing the expansion and the beans jumping during crack.
He does a great job of explaining what the difference between first and second crack.
I highly recommend this video to anyone that wants to understand more about the cracks.
I am not affiliated with Sweet Maria’s, but I do appreciate the information that they share on their website.