It actually has nothing to do with the map It's an exploit taking advantage of the selection glitch in StarCraft.
I'm on vacation right now using TV Internet, so the resoltion is horrible, expect some typos I don't notice.
- Set the latency to Extra High (it's nearly impossible otherwise)
- Burrow a Drone
- Set the Drone to a hotkey (I usually use 1 and I'll use 1 for this exaple, but you can use whichever number you wish)
- Now, select the Hydralisks
-You must press these keys consecutively very quickly; U, Ctrl 1, 1, U
- The Hydralisks should appear burrowed. Right click and they should appear out of the ground, and not have a shadow.
Wha you're doing in this is telling the Hydralisks to burrow. Then you set the Hydralisk hotkey to 1. Because of the selection glitch in StarCraft, for a short period of time, the Drone is still set to 1, but the Hydralisks will follow whatever commands are given to that unit unit. When you press 1, the Drone is selected but the Hydralisks are also considered selected (even though they aren't). if you wait too long, StarCraft successfully switches the hotkeys and the Hydralisks get selected. You press U again quickly so the Drone is told to unburrow. If you do this fast enough, this will also tell the Hydralisks to unburrow despite the fact that the Hydralisks have yet to finish burrowing. StarCraft gets confused and considers the Hydralisks unburrowed. However, because they weren't completely burrowed, the properties were switched but not switched back. Therefore, they have the lack of a shadow and the property of being walkthroughable inherent to burrowed units. This selection glitch or delay is what allowed you to morph Zerglings, Mutalisks, Dark Templar, etc., into Lurkres or Mutalisks before it was patched.
Don't tell the Hydraliks to burrow/unburrow (I forget what they have when stacked), or else the Hydralisks will repair themselves. This can be done with any burrowable unit, but it's virtualy useless with Lurkres because they have to burrw again to attack and Zerglings are so quick it takes a lot of patience to stack them and isn't really worth it.
If you wish to do this glitch again, remembre that you have to reassign the Drone to te hotkey and burrow it.
ADDITION: If the Hydralisks were pre-stacked (they pretty much start off stacked), then that's a different method requiiring the use of triggers, but I'm assuming that people were stacking their Hydralisks.