When down elevator is applied what happens to the "vortex" drag that is "assisting it's lift"?
Probably easier to to think of the "bigger barn-door" versus the "smaller barn-door". A bigger slab will be more effective than a smaller slab.
A better understanding of what the above graphic represents may be obtained by viewing the initial graphic at
post #1817 - AF447 Thread No.3. Also check out
post #1786 in this thread.
EDIT :: The graphic has been changed to show a THS airfoil with bottom camber. Also, the local Angle of Incidence is shown.