From Liquipedia StarCraft Brood War Wiki

Mutabombing consists of grouping Mutalisks with two or four Scourge using the Overlord or Larva trick mentioned earlier, then flying over a Science Vessel then attacking. The principle for Mutabombing is for the Mutalisks to take the damage (most likely dying) due to Marine/Goliath fire but keeping the Scourge alive taking the Science Vessel down almost instantly. This can be key for the Zerg because he can either stall the Terran from pushing, gaining valuable time to tech to Hive, or even destroy part of the army taking advantage of the lack of detection.