Wing Chun Elbow Force by Dr John Fung
The correct translation should be "Beneath the Elbow Force".
The power is actually not generated via the elbow joint, but referring to the Force vector generated beneath the elbow.
During Chi Sau, pushing with your forearms is wrong; trying to open your elbows is equally wrong. Pushing with your elbows, is better.
The best way is to mentally project the tip of your elbows into the opponent's space. This is similar to the projection we use when using weapons.
Imagine having protrusions that come out of your elbow and extend downwards into the opponents base - this is the feeling!