How does face-down Attack Position (via Darkness Approaches) work?
There are no oddities that you need to be aware of, but you will need to remember the technical details of each action to ensure you do not breach a rule or create a tactical blunder for yourself, as explained below.


Face-up monsters are given one (1) attack per turn. Face-down monsters are not face-up, so they cannot be used to attack. 
If a face-down attack position monster is attacked, it is flipped and stays in the same position. The damage step is worked out as if it had been face-up attack position the whole time.
No, you cannot flip-summon an attack position monster. If the face-down monster is in attack position, it does not meet the second criteria below, and therefore is not being flip-summoned. The flip-summon is


a manual change
from face-down defence position
to face-up attack position.