In SAS mode suppose you are in a hover, thus the system is continuously monitoring aircraft attitude and pilots movements, if you don't move the cyclic but the aircraft moves a few degrees from it's previous pitch and roll attitude because of an outside interference like a gust of wind, then the computer says . . . . "hey the aircraft moved, but the pilot did not move the controls, so it must be an outside interference, so I will use my actuators to return the aircraft to the position where it was before" Voila! Stabilization!!
In ATT mode, suppose you are flying straight and level and to hold straight and level the aircraft attitude is 2 degrees nose down and wings level, as soon as you release the force trim button, that aircraft attitude is "photographed" and the computers thru the actuators will hold that 2 degrees nose down and wings level attitude, which will make the aircraft more or less hold that same flight regime, it will not hold Altitude or Heading like an autopilot, but at least it will keep it still for a few seconds, in ATT mode you can make minor adjustments thru the "chinese hat" say you bump it a couple of times, that will make the attitude increase a certain amount.
A lot of newbies (Specially Robinson Newbies) to the Agusta or Bell 412 for example will fly the aircraft and keep stirring the cyclic, making a lot of control inputs, minor adjustments to keep the aircraft steady, this drives the SAS computers crazy since while the pilot is stirring the cyclic, the computers can never really get to work on stabilizing the attitude, these pilots discover that if they just stop stirring the cyclic and realize that this new aircarft does all the small adjustments for you, the aircraft becomes rock solid.