Mel - the one for the RV12 doesn't have a button - it is vernier assist
From their website
Vernier-AssistTM Throttle Controls for Dual Carburetor 912/914 Series Rotax Engines
Innovative design allows a neat, clean installation without a clunky splitter box
McFarlane invents a new type of vernier engine control
This brand new type of control has all the fine adjustment benefits of an old style push-button release vernier control, with the simplicity, responsiveness, and jam-proof safety of a friction lock control.
Light weight and compact behind the panel
Safer - no vernier threads to jam!
Patent Pending US Pat. Application #13/110,632
These light weight controls use a patent pending drive mechanism. The new roller action vernier provides smooth, jam-proof coarse and fine adjustment without a release button. Coarse adjustments are made by pushing the knob inwards or pulling the knob outwards. Fine adjustments are made by rotating the knob clockwise or counterclockwise.