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.
To make Geralt faster early on, get one of the first warrior path skills (unlocked at level 6-7 by putting points into the Witcher path). The warrior skill allows you to dodge farther distances (level 1 is %100 and level 2 is %200). While in wide open areas or even in a deserted part of a city, pull out Geralt's sword and begin to roll. This will get you wherever you want to go much quicker because, if you time it right, you can roll without pausing in between rolls. Note: If you attempt this near guards they may pursue you. Simply put your sword away as soon as they try and they will leave you alone.