Arm T-Raises work your shoulders by using the weight of your arms in combination with the speed of execution. Because you have to arrest the movement of your arms at shoulder height each time the more muscular you are, the heavier your arms are and the harder the exercise becomes.
What it works: Deltoids (the muscle group sitting atop of your shoulder joint), upper, middle and lower trapezius muscle (the muscle group going down either side of your neck and stopping about the middle of your spine).
Make it harder: You can make this harder by increasing the speed of execution. This will force you to apply more pressure to stop your arms getting past shoulder height, plus on the way down, pull them hard but stop them from slapping on your sides.