Effort is expended by pressing against the elbow pads with the backs of the upper arms; what you do with your hands and forearms is of no concern. (ironmanmagazine.com)
The Triceps are the muscles on the back of the upper arm, and below is just a brief overview of the anatomy of this arm muscle. (all-about-arm-exercises.com)
The backs of the upper arms should be pressed against the pads to ensure the only muscle used is the tricep. (mystrengthtraining.com)