It seems that when kirikaeshi is taught, especially when done slowly with emphasis on the correctness of the movement, the footwork is supposed to resemble nidan datotsu where the cut is made as the left foot is pulled forward when going forward, and as right foot is pulled backward when going backward. Kendo Shidou Yoru (official instruction manual by ZNKR) only mentions that the ashisabaki should not be an ayumi ashi with no specifics about when the cut is made, but a search on the correct footwork during kirikaeshi seems to result in what I have just mentioned.
Here is a video where Shiraishi sensei (Hanshi, 8 dan) of Marugame Budokan Isshin-kai demonstrates correct form during kirikaeshi, both slow and fast.
References
A near-replica of Kendo Shidou Yoru on kirikaeshi (in Japanese)
A recent answer to the footwork (forward) by Imafuji sensei
A Kendo World discussion around the footwork during kirikaeshi
Kenshi 24/7 article on Kirikaeshi (no mention of footwork in detail, but listed here as an instructional reference)
My take on it since Iwakabe Sensei mentioned it the last two workshops was that it was the timing of the trailing foot (forward or backwards) snapping into position as the strike was made, just like fumikomi timing, but not actual getting the stomp. The problem is when people try to get fast and start hopping. I found a good example on Youtube https://www.youtube.com/watch?v=yoyNx7qZFwo
It sounds like I may have misunderstood during keiko. I remember someone stating, when going backward, that the cut should made as the left foot steps back. If it is as the right foot is pulled back I don’t think there is any difference. Thanks for clarifying.