That glitch has been for a while where you can stack them although they aren't really good at it
. Zerglings burrow too fast so they need to be facing up left in order to stack them. It works for all burrowable units.
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As he said... Burrow one unit, Ctrl+*any number* it, select other burrowable units, rapidly press U--Ctrl+*same number*--*same number*--U. It relys on being able to give the units too many commands to do, and confusing which units...