I've been asked a few times how this works, so I'm going to start from scratch. If a movie is needed by people, I'll go make one!
Obviously, you have to be a mage. If you want to do this from the inside, you have to be high enough level to get inside the instance. However, I don't think there is a minimum level (aside from needing blink) to blink past the instance portal... If someone could test that, +rep for yoo!
What you do is head to Eastwall Gate. Y'know, the UD strat side entrance. Get past the gate and go in. Now for the fun part. You can do this either outside the instance or in the dungeon itself. Outside the instance, you will fall under the world, which will probably result in your death. Doing it inside the dungeon will let you fall underneath the instance of Stratholme.
Ok, first off, get yourself very close to the gate, you don't have to be within a millimeter of being sucked in, but if you want to try it that way, w/e!
Position yourself so you are facing the instance portal (i like to be roughly in the center, but it really shouldn't matter much), looking straight down the corridor beyond.
Do you see the blueish, shiny, candy-like Blink button?
Press the blueish, shiny, candy-like Blink button.
And voila, if all went well, you should be on the other side!
I've never managed to fall off the world without dying yet, so I can't guarantee your safety there.
Blinking from the inside (instanced dungeon) nets some interesting effects. Jump out of the hole where the gate should be, and you land at the bottom of Stratholme (the drop may be lethal, I recommend ice block or slowfall!). It's flat, I didn't see any mobs, but it's still pretty interesting. There is an area to the north where it looks like you can jump off the world, but you just hang there indefinately... until you blink out of the glitch. Mildly entertaining.
Once you're done exploring, you're a mage, just port out!![]()
Enjoy.