It's pretty simple. Kanan Jarrus gives all Phoenix allies a 70% counter chance under a Hera lead. This counter chance is individual for each time a character takes damage. The mechanic with your last example was:
Cad Bane's Turn. He uses Basic Attack.
Cad Bane attacks once and has a 65% chance to attack again
Cad Bane attacks again
Ezra Bridger has a 70% chance to counter attack each attack
Attack 1: Countered with basic. 40/80% chance to attack again. Ezra attacks again.
Attack 2: Countered with basic. 40/80% chance to attack again. Ezra attacks again.
Basically every time a character is damaged they can counter-attack, as long as they are not dazed, stunned, or have anti-counter debuffs.
