Ok, we call those movements gum-sau (pinning-hand). Jor gum-sau (left), Yau gum-sau (right), shang hau gum-sau (double rear pinning-hand) and shang chin gum-sau (double front pinning-hand). Now you see why I favor English terminology for all but the basic techniques!
As far as power comes from, based on prior comments, I'd bet that Mook's probably right, but I don't really understand what "power from the elbow" means in this situation. Since you are snapping your palm downward and straightening the arm, to some degree you have to use the muscles that straighten the elbow... such as the triceps and even the lats in the pinning striking applications. But some of these movements have less obvious applications in the lineage I studied, such as using the side gum-sau to dissolve a grapple and initiate a lateral shoulder strike to the chest. In this application, your arm snaps straight down along your side, causing the elbow to disappear and dissolve an attempted arm-grapple while your whole body flexes like a sapling adding a very powerful whipping action to your shoulder strike. In this case the power is generated almost entirely by the lateral flexing of the stance and torso. Like so much in WC/WT/VT. It's simple, but complex at the same time.
The muscles you described are correct , they are the ones you have to use.
But in our lineage mental force is located at the elbow , so mentally everything is driven from the elbow.
Its not always the case in a literal physical type of sense , but mentally the force is always projected from the elbow.
For example I can have my arm locked out straight and still use my elbow force to drop my arm down even though the elbow itself is not moving forward or back but only down .
My theory is that it came about because to mentally fire up every muscle group that is used in a Wing Chun movement would be a lengthy and complicated thought process.
So its easier to say I want my arm to go up , so I mentally send my force to the elbow and make it go up.
Same thing if I want my arm to go down , I mentally project my force to the elbow and visualise my elbow sinking down.
Our whole Sil Lum Tao form is done in this method.
But getting back to the technique that Xue Sheng was talking about , you can start the movement off by slightly sinking the elbow from the shoulder joint first , then start using your triceps to complete the movement.
The best way to learn to do it , is to get someone to physically put their hand under your hand and give you a bit of resistance , sink the elbow slightly from the shoulder first and then extend the arm down.