These are biological symbiote weapons, unique to Tyranid bio-titans. They seem to have a mind of their own, as they writhe and strike under the control of nerve bundles that are separate from the bio-titan’s main cortex.
The hulking ropes of muscle and sinew are immensely powerful, able to trap the combat machinery of opposing war engines.
Where your War Engine unit is armed with Ripper Tentacles:
When it fights against one or more enemy War Engine units in close combat, choose one Close Combat Weapons system. The Ripper Tentacles wrap around and trap that system, which means that the enemy gains no benefit from it for this close combat.