I saw it with a family member that had glioblastoma. It was weird, one of the nurses had seen it quite a few times apparently, it's also known as rallying. Family member's situation was similar to sibling comment's family, it wasn't right before death, just in the days and weeks leading up to it. It was like a sudden burst of vitality that came of nowhere, like their condition had dramatically reversed itself.

I'm sure it's a little different for everyone though.