My own experience with this specific ki test, doing it and introducing newbies to it, is that grounding is not enough - or let me put it this way: *visualizing* or *feeling* grounding, or as we might express it, mind and one point and dropping center, is not enough - for newbies esp. this results in them dropping away from the contact.
So the "extending ki" is also necessary. Not as a push back against the testing hand or leaning weight into it (you don't want to fall forward if the tester releases quickly) but as a sense of ... well different folks use different ways to express it when teaching, be it hoses with water or engaging triceps or shooting ki from fingertips. Personally I tend to explain it as meeting the pushing hand then reaching out past it, beyond the wall of the dojo to extend oneself so large as to be reaching to and inviting in the whole world.It seems to make folks get a sense of largeness they need....sort of like when I work with beginning painting students and needing to just hand them a much larger brush than they are working with!