The thing we are getting at, is if the engine is locked up. An engine could lock up because of internal damage, or just from sitting too long and developing some rust inside.
All the starter does when starting a car is turn the engine quickly - fast enough for the engine to start. When you turn by hand you are doing the same thing as the starter. You won't hurt the engine, however there is a chance the engine actually could start, and you get caught in the fan, that's why we mention to disconnect the battery (so it can't start). All you really need to turn the engine over is one or two revolutions, as long as it is easy to turn.
If you can't turn it by hand, then take the spark plugs out and squirt some lubricant in the holes (WD40, marvel mystery oil, etc) and let it sit a day. This will help loosen any rust or ridges around the piston rings. If this works, you'd probably want to change your oil after.
