I think they mean the flat face on the side of the wrench must be parallel to the strut - probably to avoid damaging the rubber boot on the ball, either by a sharp edge or by pinching the boot or to ensure that the open ended wrench isn't jammed in the joint. Note that most open-end wrenches are too thick to be used here - you may need to buy a thin wrench or grind a standard one down.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)