April, I suspect that that washing instruction is there because they can't guarantee that your particular washer has a "hand wash" cycle that is truly gentle enough, i.e. it does not agitate or spin too fast, which can cause felting, shrinkage, or stretching, depending on the material. (Is it for a knit, by any chance?) For truly feltable knits, I put them in my sink with cold water, and let them soak; I might push it around a bit with my hands but I do not wring or otherwise agitate the material at all. I can't imagine any machine cycle being this gentle and still working, the spinning at the end to get rid of excess water is probably too harsh.
For those who can't get enough water squeezed out of their sweaters after hand washing, try this:
After pushing as much water as you can out of the garment without wringing, lay a towel on the floor (or on the bottom of your tub is a good place, assuming the tub is dry). Put the sweater on the towel, and roll it up like a jelly roll. Don't worry about shaping or blocking right now. Now take off your shoes and socks and make sure your feet are clean. Stand on the rolled up towel and walk around on it, this basically uses your body weight to press the water out. After this, get a new, dry towel, lay it out where you are going to dry the sweater (I put it under the coffee table), and lay your sweater out, reshaping and blocking as necessary.
ETA: Obviously don't do the above if your sweater has breakable buttons or something like that, but most normal buttons fare fine.
