Scottish king Robert the Bruce killing English noble Henry De Bohun at the battle of bannock burn.
Robert the bruce was riding his horse up and down infront of his army before the battle, to inspire them in the traditional way. The English noble spotted this from afar and decided to take a chance, he charged at the Scottish king with his lance in an attempt to kill him from behind, before the battle even started. The king, not ready for battle yet and only armed with a small hand axe, quickly turned his horse around and killed the English knight with one blow. This of course was much more of an inspiration to his troops, who then went on to win the battle.
The painting is of the split second that could have been the downfall of his nation and his own death, if it wasn't for his bravery and luck.