Attack rearmost - how does it really work?

[BesucherXia]

It must be a popular idea to use your cavalries outflanking your opponent, kill his commanders and drive the troops to rout. But unfortunately the outflankers will often refuse to overcome the front line and attack them instead, which makes this tactic meaningless.

I remember someone said there should be a morale check for passing each hostile squard, but can anyone here explain me how does it actually work in algorithm level? Has the own morale of the outflankers influence on it? And if the enemy squard is mixed from different types of units?

Any comments will be greatly appreciated.