As Bob stated, it is a position away from center.

Also these trimmers can be used as momentary function or as selectable function on any 12X. If you purchased your 12X when they first came out and have never sent it in you will not have the momentary function and it needs to come back to Horizon for a firmware update. Only necessary if you want it.

There is an addendum regarding this here. http://www.horizonhobby.com/pdf/JR_1...l_Addendum.pdf

You can check the addendum and then check screen 83 to see if you have the updates firmware.