I heard about this some years ago. The Orcas butt the sharks onto their backs, which puts the GWs into some kind of trance. Then they just dine on the sharks at their leisure. The Orcas learned this and pass it onto their progeny as learned behavior. GWs can't be tipped over onto their backs because it hypnotizes or paralyzes them or something.
https://www.thedailybeast.com/watch-orca-whales-kill-a-great-white-shark-in-this-stunning-shark-week-footage