Continuing on with how similar "pop the cap" on the forehand is with the two handed backhand, let's look at the pull phase, where the tip of the racket gets pulled "through the slot". It's really identical to what is happening on the pro forehand, but because it involves two hands the movement is a bit more abbreviated and quicker.
It's certainly something you can't see with your naked eye when watching a professional backhand. But it's the secret to racket head speed, just as it is on the forehand.
The key here is that the wrists are doing two completely opposite movements to pull the racket through the slot. In biomechanics, those movements are called "wrist extension" and "wrist flexion".
"Wrist extension is the movement of bending your wrist backwards, with the back of your hand moving towards your body."
"Wrist flexion is the action of bending your wrist so that your palm faces toward your arm. "
In the two handed backhand, the left wrist lays backwards as you pull through the slot, so that is "extension". The right wrist curls forward, so that is "flexion". The pulling action is aided by these two wrist movements. As the hands pull forward, the extension and flexion increase.
Before "popping the cap", both wrists are in a neutral position (not laid back at all). At the end of the pull, the left wrist is fully extended backwards and the right wrist is fully flexed forward.