Mike,

I agree momentum is a beautiful thing, but unless it can be advantaged to the point of culmination, the effects on the enemy are normally transient, especially if the insurgent has a safe haven across a border where they can regroup. In Afghanistan we have only been able to leverage momentum up to tactical and operational level victories, not strategic.

Clausewitz addressed this from conventional stand point, but it applies to some extent to irregular warfare.

http://books.google.com/books?id=xym...mentum&f=false

The situation is completely different when a defeated army is being pursued. Resistance becomes difficult, indeed sometimes impossible, as a consequence of battle casualties, loss of order and of courage, and anxiety about the retreat.
Skip a couple of lines to talking about the pursuer:

The faster his pace, the greater the speed with which events will run along their predtermined course: this is the primary area where psychological forces will increase and multiply without being rigidly bound to weights and measures of the material world.