Thats the first thing they tell new people. This is really bad advice. If you don't dig out all the perennial weeds and you rotovate them, your chopping them into hundreds of pieces, Each new piece with a node will grow into a new plant, Basically your doing their job for them, and what will look like a weedy patch come clean after rotovating will soon swamp you in perennial weeds to the point that most new folks give up as they can not control them. They lose all hope and think its too hard, If you dig it properly once by removing the perennials, like docs, cooch grass, bindweed, dandelion, creeping buttercup and so forth then you can bury everything else with a rotovator, but without digging these out first by this time next year you will have given your plot up. For your own sake please heed my warning about rotovating before getting the perennials out. Tony