Many Tyranid organisms exhibit psychic powers because they act as a conduit for the awesome energies of the hive mind. Any Tyranid creature with these powers is considered to be a psyker so will, for example, be vulnerable to the Dark Eldar Crucible of Malediction. Hive mind powers do not require a Psychic test to use unless specifically stated in their description. Only those powers which require a Psychic test to use may be cancelled by enemy psykers.
This power functions continuously and does not require a Psychic test to use. Any Tyranid brood with a model within 12" of a creature with this 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 checks and tests to regroup it has to make are also passed automatically. For example, if the brood is below half strength or the enemy is within 6" it still regroups. Note, however, that regrouping only takes place at the beginning of the turn, so a Tyranid unit which falls back can be destroyed by pursuers or crossfire.
In addition to these abilities, a creature with the Synapse power is able to cause all Spore Mines within 24" to detonate in the Shooting phase. Note that this counts as the creature's shooting attack for the turn, so it may not shoot or use other psychic powers in the Shooting phase. Other creatures from the same brood may still fire at a different target if desired.