Its best to enable the option when its your turn, then you can move around in the planet map as much as you want. after you wanna end your turn, disable the option before you do.
It does modify the value, but it doesn't effect the game. i tried by changing a units max health and health to 200. ( he originally had 24 ) after taking 10 damage he had 14 health left.
i think it only works flawless with Classical turns. not sure how Cal made the trainer. think he can reply best.
If you click on the game board or on another unit, then back on the unit you are trying to attach again with, it works.