Hi! From what the others are saying, I think they mean to not be 'rough' on the cabbage--if you damage the cabbage's cell walls, they will break down and your cabbage won't be crunchy.
I thought about it--another thing might be water--you want the cabbage VERY dry. I only wash the head before chopping (although I must admit I usually buy the preshredded, as it don't feel THAT poor, but it IS more expensive!) but if you need to wash the inner leaves I'd wash them whole and then dry well before chopping.
If you can't get anything to work, perhaps it's the ingredients in your dressing and you could try another recipe.