This piece of military jargon, popularly used to describe actions in the Napoleonic Era or the 19th century generally (although it's broadly applicable for other periods) , describes the effect of a flanking maneuver on large bodies of troops.

Tactics relies on the psychology of large groups of armed men. In general a group of men will continue to fight as long as they feel that "everyone else" is fighting and the risk they incur individually and collectively may result in victory. If a group of soldiers get the sense that their fellows will run, and that the battle is clearly lost, they will typically break and flee to the rear or any other avenue where they think safety lies.

A 19th century flank attack plays on this fear, because it creates a threat of the imminent loss of the safety of retreat, making the decision to flee or not, a "now or never" proposition. Combined with the knowledge that flanked soldiers often flee, and the physical difficulty of fighting in front and on the flank at the same time (for example, a crossfire causing higher casualties), flank attacks often caused units to break.

When a series of units are placed in a line, and the outermost unit is flanked and breaks and flees, it often creates the chain reaction called "Rolling up the line". The outermost unit retreats or flees, causing the unit next to it in the line to be exposed to a flank attack, if that unit breaks, the unit next to it will be exposed and so on. Theoretically, the whole defending army could be broken in this way.

In practice, this rarely occurs. Flanks get "refused" (deliberately bent backwards) to defend, the attackers become tired or confused, the defenders can figure out what's happening and retreat in an organized way or commit reserves that stop the chain reaction.