I’m using this as well on the TBM with good results.
You have to pull your throttle back to the stop first (to “zero” or idle position). Then press and hold the button mapped to “Reduce Throttle” (it is not a toggle function). This will cause the Reverse Lever to animate backwards/down to the reverse area.
Continue to hold down the “Decrease Throttle” button and the RPMs/reverse thrust will increase. Note that the Throttle hasn’t been touched at all since it was pulled back to idle.
When you want to exit Reverse Thrust mode, release the button and push your Throttle forward a bit to completely disengage Reverse Thrust.
Hope that helps!