'leapt on his charger' implies that he attacked very quickly, perhaps after the battle had officially started but both sides were still in fact glaring at each other and getting ready. I imagine it being a bit similar to a boxing match which begins when the bell rings, but opponents will typically touch gloves as a sign of respect before actually fighting. Sometimes a fighter will use this moment to sneak in an unexpected attack and catch his opponent off guard - well within the rules, but considered poor sportsmanship.