Tyranid swarms move quickly across even the most inhospitable and broken ground under the mental lash of the hive mind. In battle, Tyranids attack on the run and often creatures are trampled, forced over cliffs or drowned by the animalistic stampede as the swarm races forward.
To represent this, Tyranids always roll one D6 more than normal when checking to see how far they can move through difficult ground (ie, three D6 instead of two D6 in jungles, and so on). However, if a Tyranid brood of eight or more models rolls two or more 6s on the dice, it suffers one wound in the stampede (saving throws apply as normal).
Synapse creatures still get to roll the extra dice but will not suffer wounds when moving through difficult ground due to their direct control of the creatures around them.