The difference between the two arms has to do with the starting balance of the stick when thrown. There are advantages and disadvantages to throwing by either arm which can be observed. To tell the difference, lay the stick out in front of you so that it resembles a moustache, as below. If you're a right handed thrower then the lead arm is on your right and if you're a left handed thrower than the lead arm is on your left. Most throwsticks are designed to be thrown by the lead arm but the Kimberley Stinger is designed to be thrown by the trailing arm, for best results.