Jib tweaker with swing arm sail control unit:
Adding a servo to the end of the jib sheet allows the jib to be "tweaked" by changing the effective length of the sheet while under sail. The challenge is to set the servo to mid point when setting the model up prior to sailing.
Tweaker with drum type sail control unit.
A jib twicher is a rig that pulls the jib boom to port or to starboard when sailing down wind.
These to devices are often confused with each other :)