All off-hand attacks have a condition that must be met or the attack will miss. If the attack misses for that reason, it wastes energy and time, but the skill will recharge instantly. While Palm Strike counts as an off-hand attack and has no requirement, it is a touch skill rather than a type of attack skill.
The most common requirement for an off-hand attack is that the assassin lands a lead attack on the target. Other requirements are landing another off-hand or dual attack, striking a knocked-down or hexed foe, or being enchanted.
The icon will appear on the right of your target's health to indicate that you have landed an off-hand attack on that target. If the attack does not connect, the icon will not appear. After 15 seconds, or if you land a lead or dual attack, the icon will disappear, and any skill that must follow an off-hand attack will not succeed.