My read.................A/TRIM
It trims the cyclic. Unlike older autopilots which just referenced themselves via the actuators against the force trim brakes until the actuators saturated (ran out of movement) the APM 2000 will reposition the cyclic via the Trim Actuators (which have an RVDT inside to tell the AP where they are.
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The Trim Actuators (parallel actuator) when in SAS/AP-SAS will re-trim the cyclic via the 4 way beep switch or release while centreing the EHA's and SEMA's when you push the FTR. When in UPPER modes of the AP the 4 way trim will change the coupled modes, L/R for HEADING, Fore/Aft (Up/Down?) for ALT, IAS, V/S as appropriate and the AP will re-trim as/if required. The A/TRIM warning serves also to tell you that there is something amiss with the upper modes and the AP is not AP - ing and you should fly it hands on.
It is not very noticeable. Most prevalent trim movement is if you couple ALT and change a big chunk of power. You will see the cyclic move. GA will make it move a fair bit as well depending on software and speed at the time.
The AP in the 135 is a bit of a mess in some ways. The SEMA's (series actuator) were installed because the EHA's (yet another series actuator) are prone to drifting over time. So you have the "basic" SAS with analog computer and EHA's then Autopilot over the top also doing SAS at a better job with EHA's with analog SAS as redundancy and then Autopilot UPPER modes with SEMA's.
SEMA - Smart Electro Mechanical Actuator
EHA - Electro Hydraulic Actuator
When you "decouple" the cyclic is always in a "trimmed" position. Saves you having to keep an eye on the "galvanometers" to see if they are reaching the stops and having to centre them by pushing the FT release.
The YAW "trim" is indicated on the PFD. Push the corresponding pedal! Takes a while to get used to as the aircraft flies left hand down and about half a ball out of trim in the cruise. Thats why your STBY AH is in the panel on a funny angle!
P.S. WTF is a BACON Relay?