Combat trim is simply a lookup table of trim settings. For example, if you are doing 200 mph and have combat trim enabled, it will go to a table and lookup your speed and set the three trims to whatever corresponds to the 200 mph entry. Combat trim is meant to keep you trimmed condition for just one condition- military power at a standard fuel and ordnance load. Change your throttle settings, drop your flaps, change your loadout, etc. and combat trim may no longer hold you in a trimmed out condition. For those conditions, you need to use manual trim to get your plane trimmed out. If you use combat trim all the time, you'll notice that when you're on final approach with power reduced and gear and flaps down, you're out of trim by a good deal and are having to put in a lot of joystick and rudder input to hold your plane at the correct attitude. If you want to get your plane trimmed out in these other conditions, you'll need to use the manual trim system when you're in those conditions.
As soon as you start dialing in some manual trim, your combat trim will be disabled. To reenable it, you either need to press the combat trim key or there is an option in the flight setup that automatically enables combat trim whenever you leave autopilot mode.
So you're on final with your power reduced and flaps and gear down and combat trim has you out of trim. You're having to push forward on the stick to keep your nose from coming up. You start repeatedly hitting the down trim key to alleviate this. As you dial in more elevator trim, you can start relaxing the amount of joystick input you are giving until the stick is centered. You would then rinse and repeat for the aileron and rudder trims and that should put you in a trimmed out condition.
What the new feature does is move all three of those trims for you when you make a single button press. Going back to the above example, how I would trim out my plane on final approach would be to hold the correct amount of elevator, aileron, and rudder input to keep the plane flying like I want it to. Then I just hit the set trim button and it looks at how much input I'm giving to each axis and begins dialing in trim to get each of those axes back to a neutral position. As it does this I just begin relaxing the amount of deflection I'm feeding into the joystick and rudder until they are neutral.
This is just an addition to the manual trim system. It doesn't replace anything. If you don't use it, you wouldn't know that anything has changed.
Although the outcome is essentially the same, trim is different in a PC sim versus a real aircraft because PC's have self-centering spring loaded sticks.
