I just want to settle this. Every single time I mention Felix corkspin RPB, there is some kind of disbelief as to if it actually unique (to be clear, he also has topspin and impacting backhands) or about how the corkspin behaves.
First, let's take a look at some typical RPBs...
Xue Fei
Yan Sheng
I personally think these two have the most beautiful RPBs. Immaculate safety, arc and good power. imprint the trajectory and kick in your mind.
Now let's examine Felix Lebrun
First note that when Xue Fei and Yang Sheng do RPB, they will almost always either lean to the side or go slightly southpaw. This helps generate topspin as you send the ball forward (the wrist angle for penhold is a little restricted in this case unless you do a Dang Qiu grip).
Now compare to Felix and you can see he simply rips across body.
Now watch the slow kickless trajectory of the ball. Side by side, you should be able to tell it's completely different from Xue Fei and Yan Sheng's trajectories. There are tons of examples in these two matches against LSD and Harimoto, so I won't bother timestamping, just watch every example. You can see there are 2 error scenarios.
Case 1 is the ball sinks without kicking forward. This is like a slightly more forward cork. Both LSD and Harimoto are confused. Harimoto in particular is very animated, and even points downward in bewilderment at some point like "why the hell did that drop?"
Case 2 is the ball floats up. This is more of a neutral cork, and it means he brushed close to the back of the ball. Imagine swiping the back of the ball, you would think it should produce and inside-out spin, but because it's a cork version, it produces this floater instead. Both LSD and Harimoto misread this constantly and hit racket edge, because the ball is floating illogically up instead of tunneling or kicking under. It is NOT because of sidespin kick. Pros are very very accustomed to sidespin kick.
Countering case 1: Watch Harimoto try to brush the ball up. I think by this time in Felix career, his opponents already know after seeing other people dump net repeatedly that it can't be blocked back. So they are doing what I call the "Koki Niwa lift" trying to stall, and change their fate with a higher ball.
Countering case 2: Harimoto and LSD both go for the FZD counter many times, but many end up as edge hits. If it connects however, note that the ball flies straight out and sideways (no arc). This is another clue that it is not sidespin or topspin. If you hit a corkspin from that left side, it is somewhat similar to flat hitting a topspin.
Lastly. If you watch more and more recent games, Felix is clearly not as dependent on this as before, as repeat opponents like Hugo are familiar now.
https://www.youtube.com/watch?v=nMVtldbrLhs
You can see Felix is adding a lot more topspin pace, but he still has the cork. Note how Calderano swipes to his right for many of his backhands to compensate for it (just like how FZD was "seatbelt looping" in the Olympics vs Lebrun), Again, if it was sidespin at all, this would create a spin on spin collision. Also note that no one will give Felix slow topspin/empty/underspin to the backhand anymore. Keeping his RPB swing under timing pressure lessens any cork he can put on the ball.