The B & C had a primer on how to move your units in 5th Ed. for the Rage rule. All you had to do was move the front models to stretch to the maximum of your movement to satisify this.
The 6th Ed. BRB Page 4:
It's interesting that it still uses the word unit, but this time the meaning is much clearer than the Rage rule from 5th Ed.:
Units must always move as fast as possible towards the closest visible enemy.
Of course, the Rage rule is completely different now.