I’ve done some testing and let myself get hit by Undie’s rocks several times, on Ascendant without altar mods.
A few observations:
- The big rock always deals damage when it hits me, but the damage is never enough to kill me directly.
- Once the big rock hits, it always spawns several fragments (5-8 according to GT) that fly into random directions.
- The fragments can immediately deal damage when they spawn, but often they don’t and I only receive damage once from the big rock.
- If the fragments deal damage, it happens one frame after the big rock dealt its damage. Also it seems highly variable. Maybe the number of fragments that deal damage is random, anything from none of them to all of them. Either way, this would mean the fragments can deal damage twice, once they spawn and once more when the player runs into them while they are flying around or lying on the ground.
Points 3 and 4 are very sus, I don’t know if that’s intentional behavior. Either it’s extremely inconsistent by design, or it’s a bug 
Going back to Afanasenkov’s video, the player loses all hp in a single frame instead of 2 consecutive frames like in my videos. Which might be explained by the beefed up difficulty of Cr190, but here’s the thing: there aren’t any fragments flying away, that’s what really irritates me. The particle effects make it kinda hard to see but it looks like they don’t even spawn at all.
I got a couple theories for that:
- The big rock directly kills the player with very high damage and the fragments don’t spawn for some reason - maybe because the player needs to be alive for that to happen? Sounds a bit far fetched tbh.
- The fragments do spawn but instead of flying away and hitting the nearby walls, they all explode and deal damage in the same frame as they spawned, killing the player with 1× big rock damage plus 8× fragment damage. Maybe the narrow space in that corner of the Crucible prevents the fragments from flying around, like they can’t compute a flight path so all of them instantly explode instead. Sounds more plausible to me.
I can’t really think of another explanation for the absence of flying fragments in that video so I still think it was a bug and the hit dealt much more damage than intended. I guess the only way to find out is to get hit by more Underking rocks in Cr190 and see what happens.