You should be using Momentum frequently throughout the story. I was using it every time it was available and had it shortly after picking up the fourth party member.
You need SP to use Momentum mode. You gain this throughout battle. It is represented by a circle next to each characters name. When it is filled to the top, a small white dot will appear above it. This represents the ability to use Momentum mode. You can build up 3 of these at once before using them. Each usage will remove one of the dots. To use Momentum mode you should select your attack or spell and as it begins being used you need to press
.