Many Dark Eldar enter battle bearing trophy racks adorned with the heads of foes they have slain. If a model with a trophy rack slays an enemy in close combat, he may chop off their head and stick it onto the trophy rack! This adds +1 to the model's Ld for the rest of the battle. Chopping off more than one head doesn't increase the model's Ld any further.
Trophy Rack
Source: Codex: Dark Eldar (2002), p. 15