All Tyranids are united by the potent psychic imperatives of their shared hive mind. Some of the larger, more intelligent Tyranid creatures have vastly expanded synapse networks within their cortex that act as a psychic conduit for the directives of the hive mind, overruling the natural instincts of nearby Tyranid creatures.
Any Tyranid broods with a model within 12" of a creature with the Synapse Creature hive mind power (including the Synapse creature itself) never have to fall back and are assumed to automatically pass any Morale checks they are required to take, even against attacks which normally force their target to fall back with no test taken. Any Pinning tests and tests to regroup it has to make are also passed automatically. Note that regrouping only takes place at the start of the turn, so a Tyranid unit that falls back can be destroyed by pursuers or crossfire.