The correct way presents itself when you try to remove the insert from the magazine with the slide inserted.
Do it the wrong way round, and you'll pull the slide out at the same time.
Hasselblads are simple things.
That's why the manuals are so thin*. And the majority of the things that are in there are so obvious that you really do not need them to be in a manual.
And that's also why it's so very easy to be an expert: you, John, will have noticed this handle thingy long ago, and never thought twice about it. Just done it right instinctively.
It's when they start putting such simple things in manuals that we begin to doubt that they really are as simple as we know them to be. Why else put them in a manual, right? But they are!
*I once gave a flippant explanation for the "M" in the Hasselblad type name as standing for "Manual", being that since the Hasselblads are that, you do not need one.
As it turned out, compared to the 500+ pages manuals that 'simple' consumer grade DSLRs of today need to explain their raison d'être, not so flippant after all.