I've had weird results on my Monk while leveling. After you do a rotation that grants greased lightning, Bootshine has a bonus of critting 100% of the time as long as you are behind the target. I've found I have to be behind the mob upon execution of the skill up until the damage number appears. If I start it from the side and move to the rear it won't crit, most of the time. If I start from the rear and move to the side, chances are it won't crit.. though sometimes it will.

Snap Punch deals extra damage from the flank, as will Twin Snakes. I've found that if I'm at the flank upon execution but move before the damage appears.. it WILL do full damage most of the time. Its only a rare occurrence that it doesn't.

This is all assuming the mob isn't turning or moving. When mobs start moving, or the tank has to turn them to avoid a cone. All kinds of unpredictable shtuff starts happening.