It does what it says - not more, not less. 10% extra when the enemy is flanked. So if you're using phantom foes (cipher lvl. 2) it's a nice item to have. Works also on non-rogues and also ranged. You can use it to give your tank or ranged chars a baby sneak bonus or pimp your rogue even further (I don't think it gets suppressed by the original sneak attack, but I'm not 100% sure).