The Spike Volleyball - S-Tier Spiker (168cm) Yongsub vs Insane Challenge Mode OASIS

Can someone explain the thumb to me, it's at about 9:07 - what is that? What caused it? Can I force it to happen? Completely random or what? Happened twice in this set and I don't recall seeing it before..... Happened after the mega whiff by the CPU at 6:39 also.... Oh we got pretty lucky at the end here too, OASIS is insane at points 18+

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