YOU NEED CERTAIN KIND OF MODEM FOR IT.
I done this many time especialy the most use out of it I get is making myself stay mounted while inside buildings.
Since blizzard stores all your game postion data on your comp, forcing yourself to lag wont cause you to warp back when your connection catches up to the server.
Here is what you do. (well what I do and what I usually use it)
Lets say your in warsong and you want to run up the tunnel but you hate getting dismounted in the middle of it, before you enter the tunnel press stand by on your modem this causes there server to stop receiving singals form you, and you press the standby button again when your almost at the tunnel. You will or might see alot of people speed run by you that means your catching up.
I never been caught or ever reported for this, people think you just lagged. I used this so much when running away from rogues lag out make them see you running straight but really you are running behind them.
I wonder if software can do the same thing, for example ZoneAlarm has a button to block/unblock all network traffic.