From Lexicanum

Bio-plasma is a Tyranid Biomorph. In 4th Edition it is generated in the stomach of a Tyranid creature. Through internal processes, a ball or bolt of blinding green plasma can be vomited at the enemy. It is often preceded by a high-pitched screaming.^1-p32 This is the reason why Carnifexes are commonly known as "Screamer-Killers" as the first Carnifexes had the Bio-Plasma biomorph.

In 2nd Edition it is a specially modified oesophagus which can vomit bio-plasmic bolts, generated by rasping internal plates, drawing its power from the stores of energy in the gut of the creature. A loud screeching noise is produced when it is launched and an electrical field around the creature's claws holds the plasma until it is fully formed. The general view is that only the Carnifex can use this, however it is available to other units for a price.^2-p58

Sources

• ¹:Codex: Tyranids (4th Edition) (identical entry in Warhammer 40,000: Wargear p40)
• ²:Codex: Tyranids (2nd Edition)