You can move up and down the stack of Frames with the Up and Down Arrow keys, the frame you are currently looking at is called the Active Frame, indicated by A1=... on the windows title line. You can also make this frame Active Frame 2, abbreviated to A2=..., by pressing '.' or using the appropriate item on the Frame menu. Now move to a different Frame and A2 will stay the same press '+' to swap between the displayed Active Frames.
2 Frame functions use both Active Frames as input and output to Out. You can Add/Subtract, Blend or Correct Brightness using these functions.
The Left and Right Arrow keys are used to adjust the mix in small steps, and 'M' and 'N' in larger steps. 'Return' terminates an operation. If you move to another Frame/Rectangle before pressing 'Return' you are given the option of applying the same change to the new area. Thus you can find the correct value by trial and error on one Frame and move to each of the other frames in turn before pressing return to apply it to all frames.