Well, I have already answered this Question very clearly in KCVDS, n i m goin to do it again here
Say If Drillroid Attacks Grand Mole in Face Down defense position
# This is a Trigger Effect
# This effect activates in the Damage Step, before a face-down monster is flipped face-up. (Therefore, this effect will not activate if a face-down "Neo-Spacian Grand Mole" is attacked. Also, if "Neo-Spacian Grand Mole" attacks a face-down Flip Effect Monster, you can return both monsters to the owners' hands without activating the Flip Effect.)
# This effect does not target.
Both Neo-Spacian n Drillroid r trigger effects that can be chained to.
If Drillroid attacks a face down flip effect monster then the monster is destroyed after flipping it face up n before damage step, the only exception to this is the case of Marshmallon where the controller of drillroid does not take ne damage of 1000lp when he attacks a face down marshmallon
If Neo-Spacian Grand Mole attacks a face up or face down(Flip or non flip effect) monster, then both the cards r returned to the owners hands before damage step n without flipping. But if mole is in face down defense position then a card attacking it will destroy mole n also inflict damage as in the case of the airknight oarshath or spear dragon etc..
Hope ur question is answered in a detailed manner