impale while underground

what triggers do u need to get a unit to burrow and then impale in random degrees and then pop out again

and

how do u make a unit only attack while burrowed. if u have played reapers arena kinda like that crab hero that can only attack while burrowed except i want the unit to impale in random places while burrowed.

thanks

cant you change the imale model to allow attacks but have the orginal model have no attacks or be enabled only when borrowed through triggers?

Zerg Lurkers, anyone? :P