Consider our earlier wrist rotation exercise: The wrist internally rotates and generates torque on the radius. That torque is valuable; it would be a waste to just let it pass through the bones of the arm like water down a drain. What we need to do is resist that internal rotation so we can store it and put it to use.
In the context of a pushup, your body does this by partnering up two functions: external rotation of the humerus and upward rotation of the shoulder blade, or scapula. These actions are managed by an army of interconnected tissues that are bigger, stronger and more capable of supporting your body weight than your arms, acting by themselves, could ever possibly be.
However, if either the wrist or the shoulder fail to hold up their end of the bargain, the muscle tissue around the elbow has to spend energy to maintain stability, rather than storing energy provided by the surrounding joints.When it works, it's a thing of beauty: The wrist drives internal rotation of the radius through gravity's acceleration. The humerus resists that rotation, storing energy in the rotator cuff, serratus anterior and oblique abdominals. The elbow becomes tightly coiled, fully extended and rock solid. And it doesn't even remotely stop there: remember, this is happening in both arms at the same time. The oblique abdominals, behemoths that they are, carry the force from their respective shoulder blades all the way down to the hip, stabilizing the low back in the process: what starts humbly in the wrist and elbow eventually rules the spinal column. In other contexts, too, like throwing, lifting and martial arts, that elbow coil acts like a force multiplier when it eventually uncoils, rapidly releasing its stored energy like a hammer drill. (If that sounds like an exaggeration, you've never seen the wing chun kung fu straight punch, nicknamed the "straight blast.") An elbow which doesn't have to spend energy to be stable is a better force conductor; it moves faster, confers greater leverage and supports more weight. Good luck! Snap a picture of your next plank and leave it in the comments!