The rule was mentioned somewhere in a discussion, I'll put a Google translate of the new French rulebook which makes it more clear, while the English version is under review:
A Hero with Command skill can give orders to the Allies on the battlefield. When the scenario indicates it, place the tile corresponding to these Allies near the Hero's card. An active Hero during the action phase of Heroes can activate the Ally units by spending gems on this tile:
- 1 spent gem activates an Allied figure that must first move and then attack. Each Allied figure can only be activated once per action phase.
- The Hero, by spending gems, can buy Additional movement points (1 gem = 1 movement point for an Allied figure) once its basic movement is made and before or after its attack. A Hero can not purchase more movement points in this way than the value of the base of movement points of the Ally.
- The Hero can also defend an Ally by spending gems. Each gem expended gives an active defense orange die to an Allied figure.
- The Hero can reroll the dice of an Ally whose result does not satisfy him, spending a gem of energy by die and by reroll. Unless otherwise stated, an Ally has only one life point. Once the action phase is complete, the gems placed on that tile are transferred to the Fatigue Zone of the Hero.