I see that OP is rotating and also bending so how did you know that his mechanics are poor?
Look at the vid. Look where on the prep phase where OP dips waist below table. Look at leg and hip movement after prep and before impact. You will see that consistently, OP does not use legs and hips.
Generating energy and transferring power vs an underspin shot requires the legs and hips as both power starters and leverage.
The OP is transferring power to the ball only with arm. This is very difficult to time and transfer power. Next Level can do it... he has had to adjust how he uses his body to generate power, because of joint issues.
Speed and spin are created from a transfer of power. The OP is not very efficiently transferring power. The fast loop vs underspin requires a good deal of power transfer... or else the shot is just a drive that relies on gravity to pull down the ball instead of spin.
I am not saying that a drive vs underspin is not a valid shot... that shot is valid and is a good option for those who cannot spin... however, making the shot with more spin allows a player to hit it harder and still land it... with even more safety.
I meant the feedback for everyone, many players do not efficiently transfer power.
The power transfer needed to make this shot explosive and safe is a SEQUENCE of muscle explosions done in order with leverage and amplification.
There are many ways to generate, amplify, and deliver power to impact. Everyone has different variations of technique, but physics do not lie. If you want a very fast and decently spinny shot vs an incoming underspin... you need to be able to generate and transfer a lot of power to the ball.
Doing that task with just the arm is poor efficiency. Next Level, who uses what looks like pretty much the arm, is actually using legs and hips as power starters and leverage... albeit using much shorter motions for obvious reasons he has explained before. His shot is a variation one he uses due to his joints. he simply cannot get down much and explode with full range of motion repeatedly, it would kill his joints.
The OP is showing a very good ability to get his hips down (meaning he bent knees and rotated hips some)… OP gets his hip level below table... One has a very good potential to generate power from that position... but OP is not using the legs and hips to start and amplify power... it is very easy to see on the vid.
The solution isn't to try to hit harder (in the same manner) but to generate the power of the shot using legs, then hips, then torso, then shoulders turn, then slow upper arm, then use forearm, then wrist, then fingers...all this if done in sequence with loose muscles used at the right time with leverage, will generate a ton of energy which can be transferred to the ball when timed right with leverage.
This power transfer should appear to a spectator to be done almost effortlessly. The spectator should be left wondering how that player just banged that ball and made it look like he wasn't half trying.
It is easy to describe these principles, but much more challenging to get an adult player to do them, especially when the player has been doing it inefficient for so long... the time required to wipe the hard drive, reformat it, reload it... that is a long process usually. Rarely happens right away. However, some players discover stuff quickly and put it to use quickly.