Overrun

A Tyranid assault maintains a fearsome momentum.
No sooner is one foe butchered than the swarm moves
on to the next kill.
Use this Stratagem when a TYRANIDS unit from
your army destroys a unit in the Fight phase, and
is not within 3" of an enemy unit. Instead of
consolidating, that unit can move (and Advance)
as if it were your Movement phase (it cannot
move within 1" of any enemy models).