It is pretty easy, and long as you are willing to sacrifice the ability to move the dial.
Create a click zone above each half of the dial. I would use a transparent shape because shapes don’t carry the baggage hotspots do. It has to have no outline, a fill, and have a high transparency (95% - 99%).
Create a shape that is placed off the stage, and give it a motion path of any length or kind, with a duration of the length of the split second you want.
The triggers are:
Add [appropriate amount] to variable [dial variable] when user clicks transparent shape on right.
Move off-stage object on motion path when user clicks shape on right
Set dial variable to 0 (or as appropriate) when motion path ends.
Of course, for the other shape, the first trigger subtracts.
If you want the dial to be useable as a normal dial, you will get involved in a more complicated dance of setting the shapes to Hidden state (to click dial), and Normal state (to reset the dial). In that case, I would recommend a separate reset button that doesn’t interfere with the normal dial behavior.