Ryan Lykos stepped away from body parts creation and presented a new Blender rig: a nice compound bow model with moving cams – the wheels that shift when the string gets taut.
Lykos duplicated the string curve for the cam and constrained an empty to it using the Follow Path constraint to hold the string on the cams, changing the offset value with drivers to move the string. Try it, maybe you'll make your own rig with this technique.