OP you are not doing it right.
The Fed slice is a superior technique comparing to the Kenny slice, in the context of the modern rackets.
1) easier face control - the chop generates centrifugal force that pulls the racket taut, so you can be extremely precise... the Kenny slice is a flip and requires more forearm strength.
2) easier timing - the high to low motion goes with gravity, the around-the-body swing path also makes the arm more connected to the body... the Kenny flat-push slice - arm extends out, can have timing problems.
3) better reach - you can throw the chop very wide or very low without losing balance, you also generate side spin to make the ball curve.... you can't do that with the Kenny slice.
4) as others said, better in handling incoming topspin.
The execution of the shot is actually quite simple.. I call it 'aim-and-cut'.
step 1 - at the top of the back swing, aim the face at exactly the intended trajectory
step 2 - cut down and push forward at the same time... you do shift weight to the front foot.
key point (many people do this wrong)... the back of the wrist is 'cupped' at the top of the back swing, but it flattens out and become slightly 'bowed' at the end of the follow thru.... this is the only way to ensure that the face looks at the exact same direction from the start to the finish.
you have this imaginary windshield... it is behind you at the top of the back swing... it is slightly open, and the racket face is on the windshield.... now you wipe it while pushing the windshield forward at the same time... when you finish the shot, the racket face is still flat on the windshield.
you can't miss with this technique.