What happens when a unit mounts the chariot?
When a rival unit’s mark is “unmounted chariot,” it stops moving when it mounts the chariot as it reached its mark.
When a player-controlled unit mounts the chariot, its Movement immediately becomes 4. This means it can likely move extra hexes that same turn. For example, if a unit’s Movement stat is 2, and it moves two hexes to get onto the chariot. It now has a Movement of 4, so it can move two more hexes that turn.