To charge head on without thought or reason against seemingly insurmountable odds and horrors no matter how bleak and dire the situation may appear, and to do so completely at ones own peril while receiving absolutely no personal gain from the action.
In combat this most likely involves giving up a superior tactical firing position for the chance to get up close and use your chain-sword on a 20 f tall monstrosity with a arsenal of fangs seemingly born from the mating of a rototiller and elephant industrial ground beef machine.
Short term unremembered glory for long term pain.
Ex - See dawn of war intro trailer on Youtube (no ones says "For the Emporer" but it is implied by their actions)
Jeff - "You hurt your back yesterday trying to lift that fridge didn't you? That was stupid man you could have just waited 1 minute for me to get there."
Martin - "For the Emperor"
Captain IronFist- sees his platoon of space marines surrounded on all sides by Tyranid forces, His comms tells him that reinforcements will arrive in minutes and he just has to hold out if he wants to survive.
Disregarding the chance to survive, He roars to his fellow brothers and draws his chain-sword in the direction of the swarm and shouts "FOR THE EMPEROR!!!" His brothers rise up with him abandoning their well fortified bunkers and heavy weapons in order to engage the largely melee based enemy up close and personal.
Soon only the Captain is left facing the mighty Carnifex with slashing blades faster then a nuclear powered lawnmower.
The Captain fights with all the power and skill of a space marine but is eviscerated right through the chest. As the beast raises him up to eye level to watch the life drain from his body the Captain shouts ""For the Emperor!!" one last time and shoves a Grenade right down the beast's throat killing both of them.
The dust settings the reinforcements arrive and the war continues and no one is the wiser.