Part of the confusion comes from your drawing is that it over simplifies the idea. For that drawing to work properly the meter would have to spin with the A and B winding with each meter lead staying in constant contact with it's AB leg. It would produce AC OR the wire loop would alternate contact with the meter leads as it spins and would produce DC. In the DC case the switching mechanism would be called a commutator. To visualize the alternator just think of the N and S poles spinning instead of the AB loop. The needle on the meter would be alternating from left to right indicating positive and negative flow. Even though the reletive rotation of the AB loop and field has not changed the North South direction flux is reversing every 180 degrees.
The direction of electron flow is reversed if either the polarity of the flux or the direction of conductor movement is reversed
The rule of thumb is called the Right hand rule of generators and there is a corresponding Left hand rule of motors and can be fully explored here:
http://en.wikipedia.org/wiki/Fleming's_left_hand_rule_for_motors
This may not be where the "rule of thumb" saying came from but it cetainly reinforced it's use.
