Acceleration and bat speed. That means a lot needs to happen at the elbow joint and usually the wrist as well. The more basic version is with just the elbow joint accelerating the forearm. It can be done with just that. But the more advanced version with the wrist and forearm is going to have more acceleration and more pop.
Ideally the blade starts slow as the ball is being contacted and you hold the ball on the rubber for a little extra but by the time the ball is leaving the rubber that acceleration (change from slow to fast) has kicked in and in that fraction of a second that the ball is on the rubber that bat speed goes from slow to wicked fast.
And you definitely have to have good feet, be in the right place, make contact close to you, in the power zone and not away from you. So there is a ton of technique that this shot takes. And I guarantee that Der_Echte would topspin any ball you give him, long or short, with his backhand.
So the technique could look right from outside to the untrained eye, but it still may not be actually the right technique. One thing to understand, if you are looping underspin and it goes into the net, no matter how you slice it, there is something you did not do in an appropriate way for the ball that you were hitting.
When you watch a good looper against a good chopper, the chopper is changing the spin, heavy, light, medium heavy, HEAVY (with capital letters), and no spin. And somehow, with a high level looper, with all those changes in spin, all the balls get looped back and the spin gets countered properly. And when you see a less skilled looper against the same chopper, some balls will go into the net and some will fly long. That second looper is not adjusting to the changing spin and therefore part of the technique is wrong.
But, never fear, practice is what helps this improve. Practice with a good coach would really help. But if you don't have access to a coach, then you need to figure out creative ways to practice this.
I do a serve and receive drill with a few friends where one person serves, the other person has to push anywhere, long, short, left, right, center, anywhere, totally random. The server's job, he has to attack whatever comes back. When you do this you start getting better at attacking long underspin and moving to wherever the ball is placed.
You could also do a drill where the push is put right to where you are having trouble until you get better at it. At a certain point you start feeling the arc and trajectory to get those pushes on the table and at a certain point you can return those long pushes with heavy topspin every time. It just takes practice. So, find a coach or do serve and receive drills or both.