During the "Prologue: Barricade" mission where you must destroy a gate by using a ballista, drop down the wall to a series of scaffold and down to the ground. You will then fight several soldiers from the rebellion group and make way to the guarded ballista. After eliminating the enemies and cleared the square, head back further back from the ballista and look to your right beside a house and you will see a dead man dressed in a white robe with a red belt (Altair Ibnu La Ahad reference) laying face down on top of a broken hay wagon. Look up to see a wooden ledge with some bird sitting around where assassin usually survey the surrounding (synchronize) and perform the leap of faith. Geralt will give a funny monologue that he commonly used in a fight that sounds “I guess they never learn", assuming that a leap from such height is impossible. You will also get a hidden ability named "Assassin" that give a damage bonus on the attack that lands on enemy's back, a reference to Assassin's Creed infamous back stab assassination.