unfortunately that portion of the tri is most active in the bottom (or top) 1/3 of most movements. IMO rope pushdowns ....flare out at bottom ...and this one you have to lock out and squeeze imo....
Also this if your gym has it and roll hands under as faras you can - hands at top as high as you can (close to cable thubs on flats): http://www.ifitnessdirect.com/images...tricep_bar.jpg