Sorry, I wasn't able to write a how-to on this new feature, so I'll give you a quick explanation here:
The action combos are used to automatically replace actions in a sequence of actions used by a combatant.
E.g. a combatant has action A (base attack) and action B (fire spell). An action combo is set up as follows:
- stage 0: required action A
- stage 1: required action A
- stage 2: required action A, replacement action B
When the combatant uses actions in order A>A>A, the combo will replace the 3rd use of A with B, i.e. it'll become A>A>B instead.
This happens regardless of how the action is selected, i.e. battle AI, control maps or selecting via the battle menu will all trigger the action combo.