Just because the losers of a close combat want to break off, it doesn't mean their opponents are willing to let them! A quick, ferocious opponent will drive home their advantage unmercifully.
To represent this, the victors can opt to roll 2D6 and compare it to the distance that the defeated troops have rolled to fall back. If it is greater, they get an opportunity to seize the initiative and keep them engaged in a sweeping advance. The unit attempting to fall back does not move and the sweeping unit makes a Pile In move as described in Diagram 9 - the unit which attempted to fall back does not do so. We assume that the cornered foe will fight back at full effect against their pursuers, so in the next Assault phase the combat continues as before, with no one counting as charging. In effect, the attackers have chosen to continue the combat instead of allowing their opponents to fall back and escape.
If the sweeping advance roll is equal to or under the fallback roll, the losers extricate themselves from the combat successfully. Make the fall back move for the losing unit - the winners can now consolidate.
Some units, like bikes and jump packers, make faster sweeping advances - typically 3D6". Where this is the case, it will be detailed in their special rules. Such units excel in harrying an opposing unit to death with their superior speed and should be employed against battered or fragile enemies for best effect. Conversely, some units, like Terminators, are too slow and ponderous to pursue a defeated enemy and are not allowed to make a sweeping advance. Where this is the case, it will be noted in their special rules.
Designers' notes: Sweeping advances have been heavily modified in order to prevent assault units rampaging across the tabletop faster than a speeding bullet. We've also taken into account that units which must make compulsory sweeping advances will tend to get wrapped up in slaughtering what they are fighting, instead of acting with the best of tactical foresight.